Bulletin STATE OF ILLINOIS DEPARTMENT OF REGISTRATION AND EDUCATION DIVISION OF THE NATURAL HISTORY SURVEY STEPHEN A. FORBES. Chief Vol. XV] I. BULLETIN Article Xn. The Bottom Fauna of the Middle Illinois River, 1913-1925 Its Distribution, Abundance, Valuation, and Index Value in the Study of Stream Pollution BY R. E. RICHARDSON PRINTED BY AUTHORITY OF THE STATE OF ILLINOIS URBANA. ILLINOIS December, 1928 STATE OF ILLINOIS DEPARTMENT OF REGISTRATION AND EDUCATION A. M. Shelton, Director BOARD OF NATURAL RESOURCES AND CONSERVATION A. M. Shelton, Chairman William Trelease, Biology John W. Alvord, Engineering Henry C. Cowles, Forestry Charles M. Thompson, Representing Edson S. Bastin, Geology the President of the University of William A. Noyes, Chemistry Illinois THE NATURAL HISTORY SURVEY DIVISION Stephen A. Forbes, Chief SCKXEPP & Barxes, Prixters Springfield, III. 1923 83820—1200 FOREWORD The present article is the twenty-third of a series of bulletins strictly devoted to Illinois River biology, containing 2108 pages and 1")5 plates, published during the past 52 years by the Illinois State Laboratory of Natural History and its successor, the State Natural History Survey, as a product of operations carried on from 1S74 to 1!)27. It was the guiding jnirpose of these studies to make a comprehensive survey of the plants and animals of the stream and its tributary waters, and to analyze their interactions with each other and with their physical environment during all seasons of the year and under the various condi- tions of successive years, esi)ecial attention being paid to food relations and to effects produced U}Kin the biological system of the stream liy the periodical overflow and gradual recession of its waters—a phenomenon of which the Illinois offered a notable example owing to its generally sluggish current and the unusual extent of its bottomlands. During the early years of this period especial attention was given to the fishes of the stream and its connected waters, but not to the exclusion of the other inhabitants. In 1894 these preliminary studies, which were a part onlv of a gen- eral program covering tiie entire state, were concentrated and adequately provided for by the establishment of a biological station, with a complete- ly portable equipment, at Havana on the Illinois River, at which place continuous investigation was carried on from 1895 to 1900, supplemented by a summer's operation with Meredosia, 4T miles below, as its center, and by a year's work on the upper river for which the station equipment was moved to Ottawa in 1901. The Illinois River operations were there- after limited for a time to occasional visits while the other streams of the state were being explored and a re])ort on the fishes of Illinois was being prepared and published, but continuous operations on Illinois River bi- ology were resumed in 1909 and were carried on with only occasional in- terruptions until 1925. The time necessary to accomplish the purposes in view was greatly prolonged by the repeated occurrence of revolutionar}- changes in con- dition, affecting the biological system of the river so profoundly as pres- ently to render obsolete nnich that had been done and to call for a repeti- tion of a considerable part of the work. The most important of these changes were, first, the introduction in ISs'.") of the European carp and its rapid multiplication, until by 1908 its yield to the commercial fishermen was greater than that of all the other fishes of the river taken together: second, the completion and opening in Januarv, 1900. of the drainage canal of the Sanitary District of Chicago, greatly increasing the amount [387] of raw sewage from the city of Chicago introduced into the Illinois at its source ; and, third, a general movement for the reclamation of the bottomlands of the river for agricultural uses, by the construction of levees to prevent an overflow of the streams and by the drainage of bot- tomland lakes. Our biological studies of the Illinois River have, of course, been carried on by the aquatic biologists of our own stafif, but for a knowledge of the ecological conditions of an aquatic situation, an acquaintance with the chemistry of the water was essential, and this has been made possible to us by the generous cooperation, at first of the Department of Chemis- try of the University of Illinois, which began analyses for us in May, 1894, and since then by the Water Survey of the State which was organ- . ized the following year. These chemical studies became increasingly important as problems of stream pollution grew in prominence and led to the addition of a chemist to the river field party during the summer season of three years (1911, 1980, 1923), to the analysis of weekly samples sent to the chemical laboratory in 1911, and to occasional trips to the river in other years by chemists of the Water Survey, as called for by the biologists ; and finally, upon the transfer of the main oper- ations of the Natural History Survey to Rock River in 1925, the State Water Survey took over the Illinois River program as a problem in river pollution, with its center of operations at Peoria, to which the Havana equipment had been transferred in 1920. The Natural History Survey still retains a permanent interest in the Illinois River, especially as a field for the solution of individual prob- lems ; and it dealt with one such problem in 1926 and 1927, when it made an exhaustive study of a new and remarkable disease of the European carp, traceable to the effects of pollution on the food supply of the carp. The Survey sustains also relations of cooperation with the Water Sur- vey, to which it furnishes a biologist whenever his services are likely to be needed in elucidation of chemical conditions disclosed. The place of the present bulletin in the series of which it is, in a sense, the final number, dealing as it does with the last twelve years of our active period on the Illinois, and discussing topics related to the whole range of our studies, may be made more evident by reference to the list of publications on the subject printed as an appendix to this paper. Stephen A. Forbes. [388] CONTEXTS PAGE Foreword, by Stephen A. Forbes 387 Collections, dates, apparatus, river levels 301 River reaches covered, main hydrographical features, and principal collecting stations [with map] 393 Load of sewage and industrial waste and zones of pollution [with chart] ' 398 Classification of the species with reference to degree of tolerance. . . 403 Some principal points regarding index value 410 Changes in the number of species taken and missing 414 Changes in the dissolved oxygen supply 420 Major changes in abundance of the more important groups i2o Changes in valuation as fish food and in per cent composition by weight 432 Temporal succession of the leading groups of pollutional or unusual- ly tolerant forms in the polluted reaches below Chillicothe 437 Competitive relations 442 Accessibility, quality, and extent of use as fish food 444 Changes in abundance of the principal groups during and just after the continuous summer floods of 1!I24 448 Comparative abundance in the channel and extra-channel areas, 1920 to 1925 4.53 Changes in the mussel fauna of Peoria Lake. 1912 to 1925 454 Explanation of general tables 458 Summary 469 Appendix 473 [3S9] VoLLMK XVII Article XII THE BOTTOM FAUNA OF THE MIDDLE ILLINOIS RIVER, 1913-1925 Its Distribution, Abundance, Valuation, and Index Value in the Studv of Stream Pollution R. E. Richardson The present paper adds the findings of two more years (1924 and 1925) to our previous accumulation* on the small bottom fauna of the middle Illinois River, and brings into comparison with the data obtained under the comparatively clean-water conditions of liUS-lill.j the results of five summers" collecting in the more or less seriously polluted bottom muds in the same territor\- between the years 1920 and 1925. The dis- solved-oxygen readings for the same or neighboring years have been fur- nished by the Illinois State \\ ater Survey. The collection of the bio- logical material in 1923 was the work of Dr. D. H. Thompson of the Natural History Survej'. The same part of the work in 1924 and 1925 was in the hands of Dr. H. J- Eigenbrodt. Especial thanks are also due to Dr. Thompson for considerable clerical work in the organization of recent data, and for valuable suggestions in the course of preparation of the manuscript. Collections, Dates, Apparatus, River Levels The statements contained in the present paper concerning the changes in the Illinois River bottom fauna during the twelve-year period, 1913- 1925, are mainly based on a total of 749 dredge and Petersen sampler collections taken between Chillicothe (14(5.5 miles below Chicago) and the Lagrange Dam (249.5 miles below Chicago). Of these, 23T hauls, with various dredges or with the mud dipper, were taken during the three years 1913-1915. The remainder, 512 collections with the Petersen samp- ler, were taken during the summers of 1920 and 1922-1925, covering five RicharrtKon, R. E., The Small Bottom and Shore Fauna of the Jliddle and Lower Illinois River and its Connecting Lakes. Chillicothe to Grafton; its Valua- tion; its Sources of Food Supply; and its Relation to the Fishery. Bull. 111. Xat. Hist. Survey. Vol. XIII. .^rt. X^', pp. 363-524. and maps; June. 1921. . Changes in the Bottom and Shore Fauna of the Middle Illinois River and its Connecting Lakes since 1913-11115 as a Result of the Increase Southward of Sewage Pollution. Bull. 111. Xat. Hist. Survey, Vol. XIV, Art. IV, pp. 33-75; December, 1921. , Changes in the Small Bottom Fauna of Peoria Lake, 1920 to 1922. Bull. 111. Xat. Hist. Survey. Vol. XV'. Art. V. pp. 327-3S8; August, 1925. . Illinois River Bottom Fauna in 1923. Bull. III. Xat. Hist. Survey, Vol. XV, Art. VI, pp. 391-422; October, 1925. [391] 392 Illinois Natural History Survey Bulletin seasons. The 749 collections mentioned were all taken either in the river proper or in the extra-channel portions of the expanded river above Peoria known as Peoria Lake, but include none from the smaller, more nearly inclosed, but connecting, bottomland lakes below or above Peoria. The distribution of these T-tO collections in time and their apportionment to river reaches was as follows : Number of collections 1913 Chillicothe to Lagrange Dam 44 1914 Vicinity of Havana 13 1915 Chillicothe to Lagrange Dam 180 1920 Chillicothe to foot of Hickory Island. 71 1922 Chillicothe to foot of Peoria Lake... 71 1923 LaSalle to Beardstown 158 " 1924 LaSalle to Beardstown 137 1925 LaSalle to Beardstown 75 Total 749 In addition to the above collections from the river and Peoria Lake there were taken in the river below Lagrange Dam in 1913 and 1915 a total of 153 hauls with dredges ; and in the connecting bottomland lakes between Clear Lake (about 10 miles above Havana) and Meredosia Bay between 1913 and 1920 a total of 406. The apportionment of these col- lections follows : Number of collections 1913 Illinois River, Lagrange Dam to Graf- ton (mouth of river) 78 1915 Illinois River, Lagrange Dam to Graf- ton ' 75 1913 Connecting bottomland lakes. Clear Lake to Meredosia Bay 113 1914 Connecting bottomland lakes, Clear Lake to Sangamon Bay 102 1915 Connecting bottomland lakes. Clear Lake to Sangamon Bay 171 1920 Connecting bottomland lakes, Liver- pool Lake to Stewart Lake 20 Total 559 Bottom Fauna, Illinois River, 1913-1925 393 Adding this group of bottom collections to the first list given, we have a grand total of 1,308 dredge and Petersen sampler hauls taken between 1913 and 1!)25 in the Illinois River and immediately connecting waters; 406 of these coming from the smaller connecting bottomland lakes below Peoria ; and !M)2 from the river proper and Peoria Lake. The usual collecting period, in all years, has been June to Septem- ber, with the larger part of the work falling in July and August. In the 1913-1915 period a small number of spring collections were taken, and in the autumn of 1914 work was extended to the end of October. The collections of the small bottom fauna in 1920 and 1922 were made without exception in the deeper open water, both of Peoria Lake and of the river, being thus confined to those areas where the effects of pol- lution were felt most fully, and where there was no unusual aeration or other protection afforded by coarse aquatic vegetation. In 1923, 1924, and 1925, a few collections were made near the margins, in the rather sparse new growths of vegetation since 1920, but the data from those lots are excluded from the figures used in valuation, and the species there taken are segregated for special treatment in the arrangement of the lists of species taken in order of index value, and their discussion. Weed and edge species have been excluded also from all 1913-1915 data used for coni]iarison. River levels during all but two of the eight seasons of collecting from 1913 to 1925 were either unusually low or about average for the warm season of recent years. The summer of 1915 was unusually wet. but ap- parently not sufficiently so to affect seriously the distribution or the abund- ance of the bottom invertebrates. The summer of 1924 had several suc- cessive floods that seem to be reflected in the bottom fauna figures for that year. For a brief further statement on river levels, and some ac- count of the effects of the floods of 1924 on the small bottom animals, see page 448. River Reaches Covered, Main Hydrographical Features, and Principal Collecting Stations The portion of the Illinois River north of the Lagrange Dam cov- ered either by the collections of 1913-1915 or by those of 1920-1925. or in both periods, has a length of 148 miles, lying between mile numbers 101.5 and 249.5 below Lake Michigan, or between LaSalle and the Lagrange Dam, respectively. This region of river breaks naturally into three main sections : the first, the 45 miles between LaSalle and the approximate head of Peoria Lake at Chillicothe ; the second, the 19.9 miles of greatly expanded river between Chillicothe and the Peoria and Pekin L'uion Railway Bridge at South Peoria, which we have assigned to Peoria Lake: and third, the 83.1 miles between the Peoria and Pekin Union Railway Bridge, or foot of Peoria Lake, and the Lagrange Dam. 394 Illinois Naturj^l History Survey Bulletin The -lo-mile reach, LaSalle to ChiUicothe, is mainly a sluggish, black-mud-bottomed section, with no important tributaries or other varying hydrographical features, except for the low dam at Henry, about three-fourths of the distance downstream from LaSalle. The effect of this dam is not important at either present or recent low-water stages, the swifter current immediately below continuing an almost negligible distance into the region of exceedingly low slope between there and Chil- licothe. The 19.9-mile section called Peoria Lake is made up, first, of two unusually wide and sluggish, mud-bottomed lakes or "wide-waters" (Upper and Middle Peoria Lake), abovtt 7.7 and 6.8 miles in length, respectively ; differing scarcely perceptibly in hydrographical character ; and separated by a wide "narrows", of visibly faster-moving water which covers a distance of around three-quarters of a mile and has mud bot- tom. The lower 5.4 miles of this 19.9-mile section (Lower Peoria Lake) is both shorter and narrower than either the Upper or Middle Lake ; has a distinctly swifter average current, relatively much wider channel and correspondingly narrower "lake" or "wide-water" portion ; and is separated from the Middle Lake, at Peoria Narrows, by a narrow neck about one-half mile in length, with unusually fast current and generally well washed, lightly silted, and Bryosoa-covercd bottom. The 83.1 miles between the. foot of Peoria Lake and the Lagrange Dam presents a great variety hydrographically, having alternating swift and sluggish reaches, a dam (Copperas Creek) about one-fourth of the way down, two tributaries of considerable size (Mackinaw and Spoon Rivers, the first important ones below LaSalle) between Peoria- Pekin and its midway point, and another still larger tributary ( Sangamon River) about 10 miles above its lower end. For purposes of conven- ience, mainly, we have broken it up, in various comparisons, into five sections ; the first four being short ones, covering only the first 40.6 miles below the foot of the Lake and ending just below Spoon River at Havana; and the fifth covering the entire remaining 43.5 miles be- tween Havana and the dam at Lagrange. The four upper short reaches recognized between the foot of Pe- oria Lake and Havana, in their turn, break into two groups at the Cop- peras Creek Dam, 23.8 miles below the foot of Peoria Lake. The upper 23.8 miles, again, has a natural break at Pekin, due to the access there of large additional factory wastes and to the marked slowing up of current that begins very shortly below that point as a consequence of the backing effect at low water of the Copperas Creek Dam. Also, the 16.8 miles between Copperas Creek Dam and Havana has a more or less natural dividing line about one mile above Liverpool, marking roughly the complete subsidence of the faster flow following the fall over the crest of the dam, and the entrance into the very sluggish deep- soft-black-mud-bottomed pool lying between that point and the Spoon River bar about 9 miles south. BoTToii FAt^TA, Illinois Rivkr, 1913-1925 395 The stretch of 42.5 miles between Havana and the Lagrange Dam covered in the seasons 1913-1915 is much more uniform over most of its length than the Peoria-Havana section. Sand, sand and shell, hard clay, or very lightly silted soft bottom is the rule throughout this stretch with the single important exception of a few miles of more heavily silted bot- tom lying immediately above the Sangamon River bar, about ."> miles above Beardstown and about 15 miles above Lagrange. The scheme of main reaches and subdivisions and a list of the principal sampling stations, or location of cross-sections, with distances below Lake Michigan, will be found in Tables I and 11, respectively. T.VBLE I Main Reiache.s axd Svbdivisions of tiik. Ii.i.i.nois Rivkr coveren by c0mp.\raiile bottom f.\una series, BOTH 1913-1915 AND 1920-1925 Reaches and subdivisions Distance in miles Upper and lower mile numbers (below Lake Michigan) Chillicothe to Foot Of Peoria Lake 19.9 146.5—166.4 Upper Peoria Lake 7.7 146 . 5—154 . 2 Middle Peoria Lake 6.8 154.2—161.0 Lower Peoria Lake 5.4 161.0—166.4 Foot of Peoria Lake to Lagrange Dam S3.1 166.4—249.5 or Foot of Peoria Lake to Beardstown 71.6 166.4—238.0 Foot of Peoria Lake to Copperas Creek Dam 23.8 166.4—190.2 Foot of Peoria Lake to Pekin 7.6 166.4—174.0 Pekin to Copperas Creek Dam 16.2 174.0—190.2 Copperas Creek Dam to Havana 16.8 190.2—207.0 Copperas Creek Dam to 1 Mile above Liverpool 7.8 190.2—198.0 1 Mile above Liverpool to Havana 9.0 198.0—207.0 Havana to Lagrange Dam 42.5 207.0—249.5 or Havana to Beardstown 31.0 207.0—238.0 396 Illinois Natural History Slirvey Bulletin Table II Location of Principal Sampling Stations (Bottom Fauna and Dissolvbh) Oxygen) Illinois River, 1913-1915 to 1925 Sampling stations Distance below Lake Michigan in miles LaSalle Spring Valley Hennepin Henry *Lacon Chillicothe Rome Foot of Partridge Island Spring Bay Spring Bay Narrows Mossville Maple Point Long Shore Beach Opposite center of Towhead Island Al Fresco Park Peoria Narrows U. S. Slips (approximate equivalent of Averyvllle of early collections) Main Street, Peoria Peoria and Pekin Union Railway Bridge... Wesley Head of Seven Mile Island Pekin Kingston Copperas Creek Dam Liverpool Havana Foot of Matanzas Lake Foot of Grand Island Head of Hickory Island Foot of Hickory Island Beardstown Lagrange Dam (77.5 miles above mouth of River at Grafton ) i-Upper Peoria Lake Middle Peoria Lake iLower Peoria Lake 249.5 • Quantitative bottom fauna collections between LaSalle and Lacon only In 1923 and after. Bottom Fauna, Illinois River. 1913-1925 397 1 f P̂ Sketch Map of the Illinois Rher, Showing Location of Principal Sampling Stations 398 Illinois Natitral History Survey Bulletin Load of Sewage and Industrial Waste and Zones of Pollution In the summer of 1914, when the small bottom fauna and the fish fauna of the central portion of the Illinois River, between the head of Upper Peoria Lake and the Lagrange Dam, were both in all their larger features for practical purposes normal, the population of the City of Chicago, more than l4o miles above the head of Peoria Lake, was estimated as 2,437,526, and the population equivalent of the wastes from animals slaughtered at the Chicago Stock Yards was approximately 869,000 persons additional. At that time Peoria and Pekin and sub- urbs had a combined population of about 86,000 and industrial wastes of unknown population equivalent, though thought to amount to sev- eral hundred thousand persons ; but all these wastes were absorbed without noticeable elifect upon the small bottom animals and without depressing the dissolved oxygen unduly. By 1920, the Chicago popula- tion is estimated to have increased around 10 per cent, to about 2,701,000 ; and the stock yards wastes, in population equivalent equal to 1,040,000, were about 19 per cent greater than in 1914, after having fallen off 353,000 since the peak of the war-time activity of 1918. Between 1914 and 1920, all of the wastes from the sources above described were re- ceived by the Sanitary Canal and Illinois River wholly untreated and subject after delivery only to the effects and processes of dilution and biological purification, varying with river levels, temperature, and other physical conditions, as chance might olTer. Between 1914 and 1920, the increase in the combined population of Peoria and Pekin and suburbs, estimated to have been over 11 per cent, but amounting to only about 10,000 in actual human units, was too small to account for any measurable part of the unfavorable changes below those two cities. During the same period, however, there is known to have been large increase in the grind of corn at the Corn Products Refining Company's plant at Pekin, so that the wastes from this plant were increased by an amount possibly almost equivalent to the combined wastes from the human population of the two towns. Other untreated wastes from the City of Chicago and environs, of which we have incomplete record, included between 1914 and 1930 that from some 300,000 so-called floating population, temporarily in hotels, etc. ; as also the wastes from the Corn Products Refining Com- pany's plant at Argo, a suburb of Chicago, and from other industries outside of Packingtown, amounting in all probably to several hundred thousand additional population equivalent. The changes in all these items between 1914 and 1920 were with little doubt upward, in unknown amount, with those at the Argo plant of the Corn Products Refining Company probably holding a leading place in importance. During this six-year period the growth of the other small up-river cities between Peoria-Pekin and Chicago, while relatively considerable, is not thought Bottom Faixa, Illixois Rivkr, 1913-1925 399 to have been important in comparison with the other sources of up- river wastes named. All of the Peoria and Pekin wastes, those from Chicago additional to the Packingtown wastes and the sewage of the residential population, as well as the wastes from the mostly small up- river centers of population between Chicago and Peoria were, as was true of the Chicago Stock Yards and residential sewage and that from Pekin and Peoria, received in the raw state, up to and somewhat after 1930. A saving feature of the situation, however, both before and since 1920, has been the fact that, with the exception of the Peoria and Pekin wastes above described, there has been at no time any important con- tribution of pollution from down-state sources along the Illinois River. Below Peoria and Pekin. in fact, with 150 miles of the trip to the mouth still vmrun, such amounts of house sewage and industrial waste as have been received have always been negligible. The changes in the amounts of waste received from the various sources above mentioned between 1920 and 1925, so far as we have any account of them, were by no means uniformly in one direction, but, so far as they affected materials received from Chicago and vicin- ity, seem to have about balanced each other in toto. These shifts evi- dently included an unusually heavy rate of increase in the city's human population, which is, in fact, currently estimated as having increased bj' •±00,000—1.000.000 between 1920 and' 1927. But to offset this increase, considerable construction of the new sewage treatment plants has al- ready been completed ; and there has been reported an 80 to 90 per cent reduction in the amount of waste received from the Corn Products Re- fining Company's .\rg"o plant, amounting by itself possibly to a few hundred thousand persons, in population equivalent. Further than the fact that the annual pack has recently continued large, we have no in- formation on changes in the volume of Packingtown wastes in the last few years. Changes between 1920 and 192.") at Peoria-Pekin. though largelv conjectural, seem since 192-t to have been on the whole in a downward direction, if we are to draw conclusions from recent changes upward in the dissolved oxygen supply at points below the Copperas Creek Dam. The movement of the population figures during these five years was of course tipward. but in too small luuubers to aft'ect the sanitary indices noticeably. An apparently much more possible source of the recent improvement below Peoria seems to be in the improvement of the methods of waste disposal used by the Corn Products Refining Company at Pekin, which, it is presiuned. made improved clarifying installations at Pekin at or near the same time it made them at Argo ( Chicago ) . The earlier of the natural reduction processes affecting the organic wastes from Chicago and its suburbs, so far as they occur in the Sani- tary Canal and the Illinois River, take place now. as fifteen or more years ago. principally in the first hundred miles oiUside of the city. 400 Illinois Natural History Survey Bulletin The location of the great septic (polysaprobic) zone, or zone of Sphacro- tiliis natans, hes within this territory, above the city of LaSalle, and does not come within the boundaries of the studies undertaken in this paper. The 136 to 1-18 miles of the Illinois lying between LaSalle and Beardstown or Lagrange Dam (the latter point located about 77 miles above the mouth of the river) has in recent years been early pollutional to late sub-pollutional ( early mesosaprobic alplia to late mesosaprobic beta in the sense of Kolkwitz and Marsson*), and is briefly character- ized by sections and dates, in the paragraphs that follow. LaSalle to Cliillicothe This stretch of 45 miles, which in 1911-1913 was early pollutional to early sub-pollutional (early mesosaprobic alpha to early mesosaprobic beta) has all been early pollutional (_ early mesosaprobic alpha) since 1920; with bottom oxygen near or at the zero point and Tiibificidae running into hundreds of thousands per sc^uare yard in the summer season. Upper Peoria Lake If some relatively small areas near shore be excepted, the 7.7 miles of Upper Peoria Lake is apparently best described as early to late pol- lutional (mesosaprobic alpha) in years since 1920. Improvement in sani- tary condition is relatively rapid in this short distance, under the influ- ence of retarded current, widely and thinly spread waters, and accelerat- ed growth of an incipient chlorophyllaceous phytoplankton, which had been held back by the conditions prevailing above Chillicothe. The same section marked the lower limit of the sub-pollutional zone (meso- saprobic beta) in 1913-1915. Middle and Lower Peoria Lake Though conditions are mixed, particularly in the lower end of this reach of 12.2 miles, the section may be described, as of seasons since 1980, as principally early sub-pollutional (early mesosaprobic beta). The improvement in the bottom muds that took place in the 6.8 miles of the Middle Lake is checked, even in the greater part of the widewaters below Peoria Narrows by wind or wave-borne local pollution from the Peoria sewers. In both the Middle and the Lower Lake the dissolved oxygen, particularly at the surface, frequently goes quite high, but is not a good index of conditions on the bottom, in recent years. This territory was, if small areas near the Peoria water front on the Lower Lake be excepted, principallv early clean-water (early oligosaprobic) in 1913-1915. * Marsson, M., Die Eedeutung der Flora und Fauna der natiirlichen Gewfisser fiir ihre Reinhaltung" sowie ihre Beinflussung durch Abgange von Wohnstatten und Gewerben. Mittlgn. d. Priifungsanstalt. f. Wasservers. u. Abwbes. Heft. 14; 1911. Kolkwitz, R., et al., Wasser und Abwasser: Die Hygiene der Wasserversorgung und Abwasserbeseitigung Leipzig, 1911, pp. 1-410; section on biology of sewage effluents, pp. 337-383, and plates. Bottom Fauna, Iixinois River, 1913-1925 401 P. P. U. Bridge to Hin'aiia This section of 40.6 miles also presents more or less mixed condi- tions : receiving the wastes of the Corn Products Refining Company's plants at Pekin, 7.G miles down ; and having a dam at Copperas Creek, at the end of the first 23.8 miles. Since 1920, conditions above the dam have ranged from pollutional to early sub-pollutional (mesosaprobic alpha to early mesosaprobic beta) over most of the area, and have appar- ently been largely early sub-pollutional (early mesosaprobic beta, in the same period below the dam. In 1913-1015 all this territory was clean- water (oligosaprobic) in the sense as usually understood. For a brief discussion of the visible increase since 1923 in the dissolved oxygen sup- ply in the portion of this zone lying below Copperas Creek Dam, (with- out corresponding improvement being reflected in the small bottom fauna up to 1925 ) see the sections on disscjlved oxygen, following. Haz'aiia to BcardstowHi The 31-mile section of river be- or tween Havana and Beardstown (cor- Havana to Lagrange Dam responding to the 42.5 miles between Havana and Lagrange, 1913-1915) seems to have Ijeen mostly late sub-pollutional (late mesosaprobic beta) from 1920 to 1923, but to have shifted strongly, as indicated by the dis- solved oxygen supply, toward early clean-water (oligosaprobic) between 1923 and 1925, though not yet very clearly so on the basis of the bottom fauna. This change, apparently largely a consequence of improvement in waste disposal at the Corn Products' plants at Pekin, is illustrated by the bottom dissolved oxygen figures in a subsequent section. All of this portion of the river was late oligosaprobic in 1913-1915. The approximate equivalence of zones from the head of Peoria Lake south in the 1911-1915 and the 1920-1925 periods may be ex- pressed finally as follows: Upper Peoria Lake in 1920-1925 about the same as LaSalle-Spring Valley in 1911-1912 (early pollutional); upper portion of Havana-Heardstown reach about the same in 1920-1923 as Upper Peoria Lake in 1913-1915 (late sub-pollutional); lower portion of reach Havana-Beardstown shifting after 1923 toward the condition of the best open water portions of Middle and Lower Peoria Lake in 1913- 1915 (early oligosaprobic). The diagrammatic summary at the end of this section illustrates clearly the favorable eft'ect of the first expansion of the river between Chillicothe and Spring Bay Narrows in checking the extension southward of the upper pollutional area between 1911-1912 and 1920-1925. Thus, the approximate boundary line between the pol- lutional and sub-pollutional zones above Peoria moved down stream not much if any more than 10 miles (or about the length of Upper Peoria Lake) between 1913-1915 and 1920; whereas the boundary be- tween the late sub-pollutional and clean-water, lying in 1913-1915 much closer to the faster current of the Lower Lake, moved more than 50 miles (or from some point in Middle Peoria Lake to Havana or below) in the same time. 402 Illinois Natural Histouy Si'hvky Bulletin CHART SHOWING SHIFTING OF ZONES OF POLLUTION Miles below Michigan 1913-1915 LASALLE 101.5 SPRING VALLEY 108.6 CHILLICOTHE SPRING BAY NARROWS 146.5 154.0 PEORIA NARROWS 161.0 Early Follutional In 1911-1912 Early Pollutional to Early Sub-pollu- tional In 1911-1912 (No collections above Chilllcothe in 1913-1915.) Late Sub- pollutlonal Early Clean- water when not affected by local sewage HAVANA 207.0 BEARDSTOWN 238.0 Principally Clean-water Clean-water 1920-1925 Early Follutional Early Pollutional to Late Pollutional Principally Early Sub-pollutlonal (See Text) Pollutional to Early Sub-pollu- tional above Cop- peras Creek Dam; largely Early Sub- pollutional below Copperas Creek Dam Principally Late Sub-pollutlonal 1920-1923 Shifting to Early Clean-water after 1923 Bottom Fai xa, Illinois River, 19131925 403 Classification of the Species with Reference to Degree of Tolerance In the comparisons later made in the present paper, illustrating changes in the composition of the small bottom fauna since liUo in the Illinois River between Chillicothe and Beardstown. seven main groups of species have been recognized in the arrangement of the various kinds in order of tolerance, as follows: I. The pollutional group, embracing seven or eight species that usually reached their highest figures 1!)'2() to i;>25 in or above Upper Peoria Lake. Here are included two genera and not less than five or six species of Tiibificidac and at least two kinds of midge larvae. II. The sub-pollutional group, unusually tolerant subdivision; fifteen species, including several each of Spliacriidac. leeches, midge lar^-ae. and Brvoaoa. These have an unusually wide range of adapta- bility under changing conditions : all of them are apparently normal to the cleaner-water zones, but also quite cajjable of subsisting, and some- times attaining very large numbers, either in pollutional or sub-pollu- tional territorv. The most important of these from the point of view of numbers is the small bivalve mollusk, Mus<:uliiiiii traiisvcrsiim. of which we have records in niulti])les of ten thousand per square yard under the widelv varving conditions of the comparatively clean lower and middle lUinciis River in i;il3-i;'15, and of the lower pollutional to upper sub-pollutional territory of Upper Peoria Lake between 1920 and 1923. This small bivalve, as well as several midge larvae and leeches, is frequently very closely associated with the more pollutional Tiibi- ficidac where they occur in greater numbers ; and it has been taken since 1920 in numbers around three hundred per square yard at points in the polluted Illinois fully 50 miles above those points in Upper Peoria Lake where the last zero oxygen readings have recently been taken as we pass downstream. III. The sub-pollutional group, unusually tolerant or doubtful subdivision. Here are included several miscellaneous midge larvae, partly incompletely determined, which had a range l!t20-192o a'.l the way from Henry ( 15..J miles above the head of Peoria Lake) to Havana and farther south. I\'. The sub-pollutional group, less tolerant subdivision, a mixed lot of more than twenty species, largely Chiroiioiiiidiic and Spliacriidac ; and also including one gastropod, one leech, a few worms, and a few dwarf or young Uitionidac. These ranged all the way from the upper end of Peoria Lake to Havana and south, under the cleaner-water con- ditions prior to 1920. but since 1920 have apparently done better under the sub-pollutional conditions between Chillicothe and the foot of Peoria Lake than in any part of the river between Peoria and Beardstown. 404 Illinois Natural History Survey Bulletin V. Pulmonate snails and air-breathing insects, five species. These are locally common, usually near the edge, or in unusual current, or in a situation combining both, in 1924 and 192.5 collections as far north as Rome (upper end of Upper Peoria Lake) ; and we have taken them elsewhere in Illinois in similar situations under conditions that can be classed only as pollutional. The normal preference of all these surface and edge forms is for clean water, and they are wholly lack- ing in index value in connection with the study of stream pollution. VI. Current-loving species other than pulmonate snails and air- breathing insects, with normal jjreference for cleaner water, but able to endure the conditions of the sub-pollutional zone in case there is un- usual current. Here are placed a dozen or more kinds in all, including two Pleuroccridae, one isopod (Ascllus intermedins), several sponges and Bryozoa, and several Hydropsychidae, the latter all undetermined, but of known habit and distribution. The index value of these species, though without question they are to be regarded for the most part as strictly clean-water forms, is poor, and their inclusion in lists without quali- fication is very likely to be misleading. VII. Cleaner-water species, about thirty species in all, including a limited number of Crustacea and Bryosoa; several snails each of the families Valvatidac, AmnicoUdac, and Viviparidac ; a few kinds of dwarf or young Unionidae; a few kinds each of immature Ephemeridae, Odo- nata, and Chironomidae ; an immature sialid ; a few immature Trichoptera; and a few adult or larval Colcoptcra. It has been convenient to sub- divide this group, from the Illinois River 1920-1925, into a less sensi- tive and more sensitive subdivision, each including about half of the total as given. Of the less sensitive subdivision we noted occasional occurrences 1920 to 1925 in the open water of the sub-pollutional zone (Middle Peoria Lake), though the majority of occurrences recorded at stations above Copperas Creek Dam in this period were from the edges. These species normally belong to the clean-water zones south of Peoria, but seem to have been largely exterminated there between 1915 and 1920, and not yet to have been reestablished in important numbers. The index value of the few occurrences in Peoria Lake recently is doubtful, because of the possible existence of springs under the lake bed there, as is known to be the case in the immediate vicinity of Spring Bay. The more sensitive subdivision of the cleaner-water group in- cludes species which have been confined in recent years to the edges or to unusual current, in cases where they do occur at all at points in Peoria Lake or elsewhere above Copperas Creek Dam. Most of these, like the less sensitive group, are recently absent or very rare in the reaches of river between Copperas Creek Dam and Beardstown, though most of them were common there, at least locally, in the period 1913-1915. Occurrences of members of this group at edges have no index value. In the following list of small bottom invertebrates, upwards of one hundred kinds (if allowance is made for several cases of two or more Bottom Fauna, Illinois Rivkk, 1913-1925 405 undetermined forms grouped together) taken in the IlHnois River since 1920 in the 13(i..5 miles between LaSalle and Beardstown are assigned places in one or another of the seven groups above outlined. Under each group, and to a considerable extent throughout the entire list, account is taken in each case of farthest northward occurrence in the more polluted sections of the Illinois River studied since 1920. Other considerations taken into account in determining the order of arrange- ment, of pollutional or unusually tolerant species in particular, have been : outside data on distribution and tolerance to pollution : association with other species of known pollutional or tolerant habit; survival under con- ditions of low dissolved oxygen supph', or where formerly-present clean water forms have been destroyed ; and, in general, relative abundance or rarity before and since the great increase in pollution in and below Peoria Lake about ten years ago. All of the records have been consid- ered in the light offered by data on the dissolved oxygen; as well as the usual or unusual physical or hydrographical factors that might be concerned. For just as the pollutional or unusually tolerant kinds may have an extreme range that carries them far outside of the pollutional or sub-pollutional zones downstream into relatively clean water, so may many of the clean-water species—under the protection of unusual cur- rent, or spring water, or proximity to aquatic vegetation, or to the margins ( where the wash, or wind and wave effects, result in mechan- ical reaeration )—advance long distances upstream occasionally into the more polluted zones. These exceptional occurrences in all the more important instances, have been given separate listing ; this is a point of especial importance in Peoria Lake recently in the case of several clean- water forms found sparingly in restricted situations outside of their general botmdaries. and likely to mislead the inexperienced worker into assuming a mtich greater degree of improvement in sanitary condition than has actually occurred over the major part of the area in the time covered by the observations. Li the complete list of species of small bottom animals that fol- lows, a half dozen of the names, among the first 12 entries, are marked with one or two stars (*;**), the latter number signifying unusual in- dex value. The six starred kinds include all taken between 1920 and 1925 in the pollutional to late sub-pollutional territory between LaSalle and Beardstown that occurred in large enough numbers to be listed as common or abundant. Brief notes concerning the index value of these six species accompany their names in the rimning list. Some further discussion of main points concerning the value of the small bottom invertebrates as indicators of pollution, as based on our recent Illinois River data, will be found in the special section on that topic next following. The relatively few cleaner-water kinds taken at open- water stations in the sub-pollutional sections between ^liddle Peoria Lake and Beardstown since 1920 were in no case present in average numbers more than negligible as compared with the abundance of the same or similar kinds in the same territory in the 1913-1915 period. 406 Illinois Natural History SrRVEY Bulletin Tablk III List of Species of Small Botto.m Animals Taken in the Illinois River, LaSalle to Beardstown, 1920 to 1925, Arranged in Approximate Order of Tolerance Classification Farthest upstream occurrence in opent water **2 3. 4. 5. **6. I. Pollutional; in general, more common in the pollutional zone than below it. **1. Tubifex tubifex. A species of unusual index value; frequently reaches very large numbers in the lower end of the septic or upper end of the pollutional zone. Limnodrilus hoffmeiateri. Likely to occur in extremely large numbers throughout the pollutional zone. Index value somewhat less certain than that of the preceding species. Limnodrilus sp. 3 Limnodrilus sp. 4 Tubifex sp. Chironomus plumosus, var., larva. Frequently occurs in very large numbers throughout the pollutional zone, though much less regularly so than L i m n o d r i I u s hoffmeisteri. Ventral blood gills vary in length as dissolved oxygen increases or decreases. Not taken by us in the septic zone of the Illinois River ex- cept at edges. Chironomus decorus. larva 8. Limnodrihis claparedianus Subpollutional, unusually tolerant; com- mon to abundant at some stations in the pollutional zone; but with original natural preference for the subpollutional or cleaner water zones. *9. MuscuUum transversiim. Extremely abundant in the' pollutional zone in company with Limnodrilus hoff- meisteri. Tubifex tubifex. and Chi- ronoTnus plumosus. No index value; equally common in some situations on clean bottom, and believed to be a case of recent adaptation. Chironomus lobiferus. larva. Occas- ionally or locally abundant in the pollutional zone; evidently has a distinct pollutional habit; but of too irregular occurrence to have great index value. Musculium truncatum Helobdella sfagnalis. Occasionally or 7. II *10. 11. *12. LaSalle LaSalle LaSalle Hennepin Henry, above dam Henry, above dam Lacon Chillicothe LaSalle LaSalle Hennepin Hennepin **:*For meaning of stars preceding names of species, see p. 405. t With exceptions noted on p. 409. Bottom Faixa. Illinois Rivf.r, 1913-1925 407 Table III — Continued locally abundant In the poUutional and subpollutioual zones; but no definite connection with pollution, as such, apparent. 13. Dina microstoma 14. Glossiptwnia complanata 15. Pisidium comprcssum 16. Tain/pus sp. 1, larva 17. Tani/piis sp. 3. larva IS. PUonateUa piinccps var. mucosa 19. Dina pari^a 20. PlnmateUa princcps var. fruticosa 21. ErpobcleUa punctata 22. Hclobdella nrpheJoidea 23. HyalcUa knickcrbockcri' III. Sub-pollutional, unusually tolerant or doubtful; species undetermined: numbers not important. 24. TanypuK sp.. larva 25. Chironominac. gen. and spp., unde- termined, larvae 26. Tanxipinac. gen. and spp., undeter- mined, Isrvae 27. Chinmumus sp.. larva IV(a). Sub-pollutional, less tolerant, more common species; normally preferring clean water; but able to stand sub-pollutional con- ditions even where the current is slight. 2S. 29. 30. 31. 32. 33. 34. 35. 36. var. lilijcasli- Pisidium. ensc Pisidium compJanatumf Sphacrium striatinum var. coipuhn- tum Campcloma subsolidum Pisidium sp. Sphacrium striatinum cnsc Sjihaerium stamincum I'rocladius concinnus. larva Cri/ptochironomus digitatus. larva IV(b). Sub-pollutional, less tolerant, less com- mon species; normally preferring clean water; but able to stand sub-pollutional con- ditions even where the current is slight. 37. Tanypus dyari. larva 38. Tanypus monilis. larva ProcJadius sp.. larva OUSJOcharta. gen. and mined Palpomyia sp., larva PlacobdcUa rugosa Anodonta imbccillis Lampsilis sp., young. Xaiididac. gen. and spp. undetermined 39. 40. 41. 42. 43. 44. 45. spp. undeter- Hennepin Hennepin Henry, above dam Lacon Lacou Lacon Lacon Chillicothe Chillicothe Chillicothe Henry, below rent dam, in cur- Henry, below, dam Lacou Lacon Chillicothe pauperciilum var. crystal- Rome Rome Rome Rome Spring Bay Spring Bay Spring Bay Spring Bay Spring Bay Rome Rome Rome Rome Spring Bay Spring Bay MossviUe Mossville MossviUe 'This species was taken as far north as Spring Valley in 1911-1912. 408 Illinois Natlkal History Survey Bulletin Table III — Continued Classification Farthest upstream occurrence in open water 46. Gordiidae. gen. and spp. undetermined 47. Orthocladius sp. 48. Lampsilis gracilis, young. 49. Polypedilum sp., larva V. Pulmonate snails and air-breathing in- sects; locally common near edges or in un- usual current, of either pollutional or sub- polhitional zone; index value none; normal preference for clean water. 50. Physa sayi 51. Planoi-hxs trivolvis 52. Arctocorisa sp. 53. Lymnaea htimilis 54. Ferrissia rivularis VI. Current-loving or edge-dwelling, purely aquatic species, able to endure conditions in the sub-pollutional zone provided there is more than usual current; numbers small; index value poor. 55. Planaria. gen. and spp. undetermined 56. Asellus intej'medius 57. Dero sp. 58. Spongilla fragilis 59. Pleurocera acutum 60. Go?iiobasis Uvescens 61. Phcmatella princeps var. mucosa- spongiosa 62. HydropsyChe, spp. undetermined (at least four), larvae 63. Vrnatella gracilis 64. Pahidicella ehrenbergii Vll(a). Cleaner-water species, less sensitive group; occasional occurrences in open water in the sub-pollutional zone, but usually taken 1920 to 1925 only at or near edges in Peoria Lake and at other stations above Copperas Creek Dam; these species nor- mally belong to the clean-water zones, be- low Havana, at present, but are rare and scattering even there since 1920. Index value of occurrences in open water in Peoria Lake doubtful in several cases, be- cause of possibility of existence of springs under the lake bed. (See also pp. 454-458, on Unionidae.) 65. Valvata trioarinata 66. Valvata Mcarinata var. normalis Long Shore Beach Wesley, strong current Wesley, strong current Seven Mile Island, strong current Rome Rome Peoria Narrows, strong current Peoria Narrows, strong current Peoria Narrows, strong current Spring Bay, current Spring Bay, current Peoria Narrows, current Peoria Narrows, current Peoria Narrows, current Peoria Narrows, current Peoria Narrows, current Peoria Narrows, current McKinley Bridge, current McKinley Bridge, current Mossville *Mossville Bottom Fai:n.\, Illinois Rivkr, 1913-1925 409 Table III — Coiicluded 67. Carnis sp. 1, nymph 68. Valvata hicarinata 69. Vivipara contectoides 70. Amnicola emarginata 71. Sialis infumata, larva 72. Pectinatella magnifica 73. Pluviatella polymorpha var. repens 74. Gomphus plagiaius, nymph 75. Gomphus crtcrnus. nymph Vll(b). Cleaner-water species, more sensitive group; occasional occurrences, at edges or In unusual current only, at stations between upper end of Middle Peoria Lake and Cop- peras Creek Dam. Most of them still absent or very rare in the reaches between Cop- peras Creek Dam and Beardstown, though most of them were formerly common there at least locally. Index value none when they occur at edge. 76. Tropisternus dorsaUs. adult 77. Ischnura verticalis. nymph 78. Lampsilis parvus 79. Vivipara suhpurpurca 80. Lioplax siibcarinntus 81. A»(ix jiiniiis. nymph 82. Rhyiicophila sp., larva 83. Amnicola limosa 84. Enallagma signal um 85. Sotnatogyrus suhglohnsus 86. Leptoceridae, gen. and spp. undeter- mined, larvae 87. Palacmoiirtes exilipcs 88. Hcxagenin bilifieata. nymph 89. Cordylophora larustris VII(c). Cleaner-water species, more sensitive group; not taken above Copperas Creek Dam at all in years since 1920; and only scatter- ing occurrences below Copperas Creek Dam in recent period; the last named, Chirono- mus fcrrtigineovittatus, was very common in Illinois River black muds as tar north as Copperas Creek Dam in 1913-1915. 90. Truncilla donaciformis 91. TruneiUa eJegans 92. Polycrntropus sp., larva 93. Molannidae, gen. and spp. undeter- mined. 94. Chironomus ferrugineovittatus, larva *Mossville *Maple Point *Maple Point *Maple Point *Long Shore Beach *A1 Fresco *Wesley, current *Kingston *Kingston tRome tRome tMossville fMossville tMossville tMossville tMossville tLong Shore Beach tAl Fresco tPeoria Narrows, current tPeoria Narrows, current tPeoria Narrows, current :j;tPekin, one occurrence, 1923 tKingston, one occurrence. 1923 Havana Matanzas Copperas Creek Dam, below Havana Matanzas * Farthest north in open water. t Farthest north at edges or in unusual current. t This was the sing"le occurrence in collections above Havana between 1920 and 1925; was taken in 1S13-1915 as far north as Upper Peoria Lake. 410 Illinois Natural History Survky Bullk.tkv Some Principal Points Regarding Index Value Various extensive published lists, as well as questions frequently asked by workers newly interested, seem to imply, to say the least, an overconfidence in the simplicity and efficacy of the use of a few or many so-called index organisms in the determination of degrees of stream pollution. Less frequently are we asked to name those kinds, particularly of small bottom organisms, which are likely to be most useful for that purpose. This question often appears to be a definite reflection of the fact that various published lists, including our own, are much too long to be useful to the uninitiated worker without a good deal of explanation ; as both it and other variations of inquiry seem to result from an impression that biological determinations of the extent of injury by sewage or other waste can be made by a more or less rule- of-thumb mechanical method, the practice of which calls for little by way of preliminary knowledge, except the names and identity of the species. As a matter of fact, the number of small bottom-dwelling species of the fresh waters of our distribution area that can be safely regarded as having even a fairly dependable individual index value in the present connection is surprisingly small ; and even those few have been found in Illinois to be reliable as index species only when used with the greatest caution, and when checking with other indicators. The septic zone is, however, as compared with the pollutional and sub-pollutional zones next below it, much the more easily recognizable, whether by chemical determinations, by its physical appearance and its odors, or by a limited number of characteristic organisms ordinarily found in quite large numbers in various situations within it. But. as il- lustrating the lack of fixed rules even in this zone, the most abundant and characteristic plankton species of all the septic kinds taken in the upper Illinois River in 1911-1912, Sphacrotihis uataiis, was wholly ab- sent from the middle and lower end of the Chicago Sanitary Canal, where examined the same seasons, although those waters were and are also septic. Again, Tuhifcx tiihifex and other associated Tubificidae, com- monly regarded as characteristic of the septic zone, were distinctly most abundant toward the lower rather than the upper end of the septic zone of the upper Illinois in 1911-1913; while in the lower end of the Sani- tary Canal, also septic, they were wholly absent in all bottom dredgings in those years. The most serious limitations on the use of the members of the small bottom animal population as indices in the pollutional zone, which is at the same time unusually difficult to recognize either from its physical or chemical features, have to do both with their frequently very confusing latitude of distribution and with the fact that so few of them occur in numbers large enough to encourage their individual use as indicators. As an actual example, it is found that out of a total of more than 87 kinds of miscellaneous small bottom animals taken in the pollutional zone between LaSalle and the foot of Upper Peoria Lake in the four Bottom Fauxa, Illinois Rivkr. 1913-1925 411 years of collecting between 10"20 and I!)-'.') only two. that is, one tiibificid worm (Lnnnodnhis lioffmcistcri) and a single lar\al chironomid (Chi- rononius plumosus I , could be said to have been generally coninion enough over wide ranges and to have fulfilled at the same time the other re- quirements necessary to encourage moderate confidence in their value as indicators when taken by themselves. But of these at least one. the pollutional chironomid, Chironomus plumosus. has usually been classi- fied heretofore as septic; as has apparently also been the case a good deal of the time with Liiiiuodriliis hoffuicistcri. a species very easily con- fused, in the absence of laborious microscopic examination, with Tiibifc.v tubifcx. A second pollutional or unusually tolerant chironomid larva, Cluronoinus lobifcnis. occasionally has occurred recently in rather large numbers in the middle Illinois River, but at such widely separated points as to remove it from consideration as an important index species. The single remaining species, of the 2T kinds mentioned above, that has re- cently shown great abundance over wide territory, the small bivalve mollusk, Mnsciilii(iii transvcrsiiiii. in its turn, cannot be regarded as hav- ing any inde.x value at all. Although necessarily listed as an occasional pollutional species, because of its close association with Liiiniodriliis hoff- mcisteri and Chiroiiomns plumosus in pollutional muds above Peoria, it is correctly regarded merely as a case of tmusual adaptability in a form that nonnally reaches quite as large numbers under clean-water condi- tions as those recently recorded by us in the pollutional territory of Pe- oria Lake and the neighboring parts of the Illinois River. Still confining ourselves for illustration to the pollutional zone, and assuming that both Limiiodrihis lioffmcistcri and Chironomus plumo- sus have been recorded as present, we may inquire what standards, if anv, are to be followed in striking the boundary line between numbers that are important and numbers that are best disregarded. It is as well to say at once that the question cannot be answered : for the inter- pretation of degrees of abundance, both of individual species and of small groups of kinds with similar habit, is extremely likely to be a whollv relative matter. Thus, in l!)"23-in25. we found the combined tubificid totals per square yard varying from under 1,000 to over 350,000 in the pollutional territory above ChilHcothe at individual sta- tions without having any ground for supposing conditions better at the one class of stations than the other. Likewise we have instances where Chironomus plumosus varied from near zero to more than one thousand per square vard in the same territory in the same season or between two seasons, without any evidence of change in sanitary condition appearing in the interval. Floods may carry away eggs or young midge larvae ; severe winds may blow away swarms locally after emergence but be- fore egg-laying ; or bottom sampling may be done when the stages pre- sent are too small to be recognizable by the ordinary methods of re- covery employed. On the other hand, numbers, whether of worms or midge larvae, that mav appear low in comparison with some of the low- est we have mentioned may be significant of serious change in sanitary 412 Illinois Natiral Histohy Survey Bulletin condition when compared with average previous rates of occurrence of the same forms in the same area. Individual species quite unusable alone for various reasons as in- dex organisms frequently acquire a cumulative value for that purpose as they come to be grouped together, particularly when there are lists of former inhabitants of the same area under presumably cleaner-water conditions for comparison. Here kind is very likely to become more important than numbers, and a knowledge of the previous history of the same or similar areas more important than any number of previously compiled lists of so-called key organisms graded according to index value. A good proportion of the conclusions presently drawn from the study of our recent Illinois River data are based upon this sort of grouping, as opposed to individual index value. Not infrequently absence or much reduced numbers of formerly present clean-water species in an area may be quite as important or even more so than numbers of known pollutional forms found in de- termining degree of present or recent pollution. The pollutional forms themselves may be largely excluded by the nature of the original bot- tom, as was recently the case in several short hard-bottomed reaches of the Illinois River only a short distance below the foot of Peoria Lake. Still again, there may be other special invisible excluding factors, as toxic factory wastes, operating against the successful entrance of pol- lutional species in normal numbers into a polluted area. And, as a con- cluding illustration, in essentially late sub-pollutional territory, the con- dition of the bottom may be fairly good over a large portion of the year, and the absence or scarcity of cleaner-water forms may indicate the periodic incursion of pollution with sudden or prolonged increase of water levels. When a good supply of pollutional forms are present, on the other hand, the fact of absence of formerly present clean-water kinds may have considerable value as an additional check. And in the absence of any knowledge of the previous history of the same area, lists of species from similarly conditioned and located unpolluted territory may serve, to some extent, the same purpose. An almost inextricable confusion of all zones from septic to clean- water is frequently met with in very shallow streams supplied with vege- tation during the heated season, though scarcely less so than may some- times occur very close to the margins of some large lakes and rivers. Herein lies the explanation of the comparatively rapid rate of self- purification found by Weston and Turner* in the Coweeset River below Brockton, Mass. ; and by ourselves in 1914 in the Fox River below Aurora and below Elgin, where the transition in mid-channel, in each case be- low a dam, from late septic or early pollutional to practically a clean- water fauna was accomplished under midsummer low-water conditions in a distance of hardly more than 3 miles. In such very shallow areas • Weston, Robert Spurr, and Turner, C. E. Studies on the digestion of a sewage filter effluent by a small and otherwise unpolluted stream. Contribution from San. Research Lab., Mass. Inst. Techn., vol. X, pp. 1-96; 1917. I Bottom Fauna. Illinois Rivkk. 1913-1925 • 413 the rate of reaeration from the plants customarily results in long con- tinued supersaturation ; and the various oxidizing and reducing processes, as well as the growth of the attendant organisms, arc no doubt further accelerated both by the higher temperatures and the better access to light supplied. Because of all of the various complexities above mentioned, and others, including those introduced by shifts from one to another dis- tribution area, it can be seen that the individual student of the biological side of stream pollution in a new locality is bound sooner or later to be forced back upon his own resources to a large extent. He is very likely to find, in fact, that it is only after he has worked up his own species lists and arrived at his own conclusions as to index value and interpretations based upon it, whether as affecting individual species or groups, that previously published data from outside areas begin to fall into place and to serve a really practical use for final checking and comparison. While certain strictures on the value of dissolved-oxygen readings as indicators are made in this paper, there has been no intention unduly to minimize the value either of that or the other usual chemical indices. The cases noted as calling for particular caution are those of lag of the bottom condition behind that of the plankton and the oxygen supply. These are most frequent in streams where there is a sudden and marked slowing up of current that continues long enough to permit the rapid niulti]ilication of chlorophyll-bearing plant and animal plankton, w-ith- out allowing a permanent and parallel im])rovenient on the bottom in the same time over the same ground. Such instances aside, it must be said frankly that the simple procedure of listing side by side our recent dissolved-oxygen readings and the farthest upstream occurrences of our various Illinois River bottom species from the unwidened Illinois River and Peoria Lake channel both above and below Peoria has served as one of the most important general sources of aid in getting order out of the chaos that seemed to reign in all directions when the unorganized data were first spread out for study. Both for that and for other reasons the writer is strongly of the opinion that dissolved-oxygen determina- tions should hold a fixed place as accessory routine in all biological studies of stream pollution. If the problem set involves nice determitiation for the first time of the boundary lines between zones in the Kolkwitz-Marsson* schedule of self-purification, figures for free ammonia and nitrates, particularly if expressed as percentages of total nitrogen, also will be found of value. Because of the wide range of error due to the variable mortality of the less pollutional plankton organisms during the incubation period and to other interfering factors likely to enter at any point below the septic zone, the usefulness of bio-chemical oxygen-demand determina- tions is quite likely to prove doulitful except as a test of the strength of raw sewage or relatively young effluents. • For bibliographical references, see p. 400. 414 Illinois Natural History Survey Bulletin Changes in the Number of Species Taken and Missing Reduction in the total number of Severe downward changes in different kinds taken bctvjccn 1913- the total number of kinds of small 1915 and Ipso. bottom animals taken occurred in all sections of the river and Peoria Lake between Chillicothe and Beardstown between 1913-1915 and 1920. In the three subdivisions of Peoria Lake the reductions ran in all cases over 50 per cent : being 69 per cent in the Upper Lake, 72 per cent in the Middle Lake, and 50 per cent in the Lower. The largest percentage and absolute reduction of all, 83 per cent, occurred in the approximately 41 miles between the foot of Peoria Lake and Havana, where the total num- ber of kinds taken dropped from 91 to 15 in the five-year period. In the 31 miles between Havana and Beardstown there was a decrease in the same time of 48 per cent, or from 43 to 23 kinds. The largest decrease quite naturally took place in the section of river between the foot of Peoria Lake and Havana, where both the total number of all kinds and the number of cleaner-water kinds had been highest five years previously. The percentage losses in Upper and Mid- dle Peoria Lakes were also not much less (69 and 72 per cent) than be- tween the foot of Peoria Lake and Havana, though the absolute losses were conspicuously less, because of previous contraction of the lists in re- sponse to mild pollutional conditions that prevailed before 1920. The siz- ably smaller percentage loss in the Lower Lake (52 per cent) was no doubt in great part due to the much better protection afforded by the unusually rapid current that prevails over a large part of that area. The smallest loss of all, 48 per cent, in the section next below Havana, was probably due both to the tapering off of the pollution with distance and to the rather better average rate of current in a large portion of that sec- tion than in the 41-mile section just above. Reasons why the reduction of the lists was not even more complete, particularly in the Peoria Lake region, and in the sections of river more immediately below Peoria, are to be found chiefly in the fact that in all of the subdivisions considered between Chillicothe and Beardstown a rather large but varying number of species were, even as early as 1913- 1915, of such kinds as we might expect to show considerable tolerance. A table showing the total number of pollutional, ususually tolerant, and tolerant kinds contained in the 1913-1915 lists is given on page 419. Those figures show that 38 to 51 per cent of the total number of species present in the three sections of Peoria Lake in 1913-1915 were assignable to either one or another of the pollutional, unusually tolerant, or tolerant groups : while in the section Wesley to Havana the percentage was 24, and in Havana-Beardstown it was 32. The increases in the total number of all kinds of small bottom species taken in the various reaches between 1920 and 1925 were due principally to increases in the more tolerant kinds, and are discussed in the sections immediately following. Bottom Favxa, Illinois Rivkr, 1913-1925 415 Table IV Total Numrer of Species Taken, ExcLrsivE of Eiwe Forms and Those Pro- tected BY Unusual Current Reaches 1913-1915 1920 Per cent decrease 1915 to 1920 1924 1925 Upper Peoria Lake 33 Middle Peoria Lake 36 Lower Peoria Lake 44 Foot of Peoria Lake to Havana 91 Havana to Beardstown 43 10 416 Illinois Natural History Survky Bullktin In the combined stretches of river between Peoria and Havana the number of clean-water kinds of small bottom animals taken varied rather widely between 1920 and 1925, but ended in 1925 with only -i as compared with 3 in 1920 and 49 in 1913-1915. In the section be- tween Havana and Beardstown, rather similarly, the number of clean- water kinds taken actually dropped between 1920 and 1924, and had risen only negligibly in 1925 as compared with 1920. It is of course to be kept in mind that the persistence of a small number of kinds is not so conclusive of continuing unchanged pollution, as the sharp re- ductions between 1913-1915 and 1920 were of its incidence; since it is quite within the possibilities for improvement to occur (as we think it has since 1923 below Copperas Creek Dam) at a visibly more rapid rate than the locally exterminated species are able by natural means to reestablish themselves. Table V NuJiiJER OP Clean-water Species Taken, Exclusive op Eiige-forms and Those Protected by Unusual Current Reaches Total of Bottom FArNA, Illinois River. 1913-1925 417 three subdivisions of the Lake, where there were already in 1913- 1915 relatively large numbers of tolerant or unusually tolerant kinds, the wave of pollution originating at Chicago was heaviest ; while in the first 40 miles below Peoria, where the combined Chicago and Peoria load was probably on the average somewhere near the same, there was a visibly larger percentage of clean-water and less tolerant kinds. The smallest of all the missing lists, that in the 30 miles between Havana and Beardstown. which stood at 2T in 1920, representing a loss of 62 per cent from the 1913-1915 total, was itself far from small. The moderately better showing made there in 1920 than in the sections of river immediately northward was of course largely a matter of the greater distance from the sources of pollution upstream. The rate of replacement of missing species between 1920 and 1925 was fairly uniform over the three sections of Peoria Lake, and was more rapid in all three of them than in the next 70 miles of river below Peoria. Thus, the portion of the 1913-1915 lists missing dropped in the three subdivisions ( L'pper, Middle, and Lower Peoria Lake) from 'ItS per cent, SS per cent, and 72 per cent to 5T per cent, 63 per cent, and 63 per cent, in order downstream. In rather sharp con- trast, in the 40 miles between Peoria and Havana, the percentage missing dropped between 1920 and 1925 only from 85 per cent to 79 per cent. The difference in rate of replacement between this section of the river and Peoria Lake is clearly a consequence of the larger proportion of relatively sensitive species and the smaller percentage of unusually tolerant kinds found here in recent j'ears. together with the naturally slower rate of reestablishment of the more sensitive forms. In the lower- most section of the river studied 1920 to 1925. Havana to Beardstown, as in the reach Peoria- Havana, and apparently in large part for similar reasons, the size of the missing list changed hardly noticeably during the five-year period, the percentage of the 1913-1915 list missing standing at 5S in 1925 as compared with 6'i in 1920. T.\BLK VI Total Nvuber of Species fko.m 1913-1915 Lists Missing 418 Illinois Natural History Survey Bulletin When we add together, for Tin- cliangcs in the total number of each section of the river and Peoria pollutional, unusually tolerant, and Lake studied, the poUutional, un- tolcrant kinds taken. usually tolerant, and tolerant kinds of small bottom animals taken in the 1913-1915 period of relatively clean water and compare with the totals of the same groups for 1930, immediately following the severe wave of pollution that destroyed most of the cleaner-water species, it is surprising at first to note that with but one short reach (Lower Peoria Lake) ex- cepted, these numbers decreased also, rather than increased, as the clean- water species declined. Thus the total of pollutional, unusually tolerant, and tolerant kinds fell from 17 to 10 in the Upper Lake between 1913- 1915 and 1920; from 15 to 10 in the Middle Lake; from 22 to 12 in the section between Wesley and Havana ; and from li to 10 in the section between Havana and Beardstown. The only increase occurred in Lower Peoria Lake, where the total of pollutional, unusually tolerant, and toler- ant kinds rose from 17 to 20 between 1913-1915 and 1920. Next we notice that between 1920 and 1922, as conditions on the bottom became slightly, but only slightly, better, the total of pollutional, unusually tolerant, and tolerant species rose sharply in the two upper sections of Peoria Lake, and moderately (20 per cent) in Lower Peoria Lake. These increases were added to in all three sections of Peoria Lake between 1922 and 1923; and the change between 1923 and 1925 left the figures either larger or negligibly less than in 1922. In the river between Peoria and Havana also the total number of pollutional, un- usually tolerant, and tolerant kinds nearly doubled between 1920 and 1923, dropped back again close to the 1930 figure in the severe flood summer of 1924, but rose again to the 1923 figure in 1925. In the section between Havana and Beardstown there was the least increase between 1920 and 1923 (only 10 per cent), but after remaining un- changed through the severe flood of 1934, it nearly doubled in 1935. The first reason to be noted in explanation of these changes is that neither the various sections of Peoria Lake nor the sections of the river proper studied between Peoria and Beardstown were even approximately clean five to seven years before 1920, though the latter two sections or at least the last one were relatively much more so than the lake. Sec- ond to be noted is that not merely the unusually tolerant and less toler- ant species of our presently adopted classification, but also most of the pollutional species have an exceedingly wide range of distribution, which normally carries them, at least in moderate or small numbers, into rela- tively clean-water territory. In other words, the pollutional, unusually tolerant, and tolerant species were already there in 1913-1915, to de- crease or increase as conditions might warrant. So, in the Upper Lake the combined number of pollutional, unusually tolerant, and tolerant kinds made up 51 per cent of the total of all kinds; in Middle Peoria Lake it made up 41 per cent; in the Lower Lake 38 per cent; and even Bottom Faixa, Illinois Rivkr. 1913-1925 419 a .J < < H H Z, < » a o c < Z < a o 1-1 < < z; o o PL, 1^ .5 CD 420 Illinois Natural History Survey Bulletin in the reaches between Peoria and Havana and Havana and Beards- town, it made up 2-1 and 32 per cent of the totals of all kinds taken. The noticeable decreases in the total number of pollutional, un- usually tolerant, and tolerant kinds of small bottom animals recorded as pollution increased between 1913-1915 and 1920 in all sections of the river here compared were naturally greatest in the two last-named groups of the three. The very marked rise in the abundance of the Tubificidac in Upper Peoria Lake after 1920, as the condition of the bottom muds remained substantially unchanged or improved but slightly, however, suggests that even some of the pollutional kinds may at times be subjected to periods of low oxygen that are longer and more severe than they are equipped to stand. The good increases in totals of pollutional, vmusually tolerant, and tolerant kinds between 1920 and 1925 are believed to suggest improve- ment in the condition of the bottom muds as compared with conditions at a hypothetical apex of pollution shortly before 1920 rather than in that year. And the relatively slight improvement on the bottom indicated by the failure of the cleaner-water kinds of small bottom animals to in- crease and of the supply of bottom dissolved oxygen to improve above Copperas Creek Dam since 1920 is further confirmed by the fact that in all reaches above Beardstown the increase in the total number of kinds of small bottom animals taken was almost equalled or exceeded during the same five years by the increase in the combined number of pollutional, unusually tolerant, and less tolerant kinds. The cases (of Lower Peoria Lake and Havana-Beardstown) where the increase in pollutional. unusually tolerant, and tolerant kinds exceeded the increase in all kinds in the five years following 1920 result from the actual de- crease in the total number of clean-water kinds taken in those two reaches during the period. Changes in the Dissolved Oxygen Supply Sharp decreases in the surface In July-September. 1911 and dissolved oxygen in mid-channel 1912, the nearest years to 1913- bctiveen Chillicothc and Beards- 1915 for which we have records, tozvn bctiveen igii-1912 and ig^o. the dissolved-oxygen supply at all points between Chillicothe and Beardstown. as measured by surface figures in mid-channel, ruled moder- ately to well above the usually accepted minimum point for most of our fishes (or above 2.50 parts per million). Between 1911-1912 and 1920, the largest decline occurred at Chillicothe, where average figures of 3.72 and 3.0 parts per million in July-September, 1911-1912, gave way to an average of only 0.47 parts per million for four readings taken July-Sep- tember, 1920. The readings obtained in the three sections of Peoria Lake in the summer of 1912 are so scanty, and so irregular (due prob- ably to the coincidence of some of them with rich and some of them with poor phytoplankton periods) that they are of almost no value for com- parison and are best here omitted. Dropping only ten miles below Bottom P\\una, Ii.li.nois Kivkk, 19131925 421 Peoria to Pekin, the downward trend was clearly resumed, the decline there between July, 1912, and July-Se])tcml)er. li>2(i, having been "2,4 parts jier million, or from 5.1 to 3.0 parts per million. Above the Cop- ])eras Creek Dam the compari.son between July, 1912, and July-Septem- ber, 1920, was even more sharp, the figures dropping from 4.0 to 1.25 parts per million. At Havana there was a slump from 3.65 parts per million in July. 1912, to 2.25 in yulv-Sei)tember, 1920: and at Beardstown from 4.8 to 2.35. T.M-.I.K VIII Changk.s in Surface Dissolved Oxygen, Mii)-ciia.\.nel. Chillicothe to Beards- town, 1911-1912 TO 1920; Parts per MiLLi0N*t Sampling stations 1911 July-Sept. 1912 July 1920 July-Sept. Chillicothe Upper Peoria Lake 1 Middle Peoria Lake I Lower Peoria Lake J Pekin Above Copperas Creek Dam. Havana Beardstown 3.72' 3.0 .see text. 5.4 4.0 3.65^ 4.8 0.47' 3.0' 1.25^ 2.25' 2.35 • Exponent is number of readings averaged. t Water levels, all years, about normal for season. Slight or uupcrccptlblc increases ill bottom dissolz'cd o.v\'(/ni fifiiiri'S ill Upper and Middle Peoria Lake channel since IQ20. in general, the evidence af- forded by the bottom dissolved oxygen figures from the channel indicates very little change either way in the sanitary condition of the bottom muds between Chillicothe and the foot of the Middle Lake between 1920 and 1925. If recent estimates* of increase in the popu- lation of the city of Chicago between 1920 and 192T are at all to be relied upon, however, the persistence of low dissohed oxygen figures in this portion of the channel, in spite of recent enlargement of the fraction of the total population taken care of by the new sewage-treatment i^lants, can hardly be regarded as surjirising. For the purpose of the present comparisons we have taken a selec- tion of the micKchannel stations covered in the summer work of 1920, 1923, and 1925; leaving out 1924 because of the abnormal diluting effect of the continuous summer-flood of that 3'ear. Taking Chillicothe and Rome as rej^resenting Upper Peoria Lake and vicinity, it is true that average bottom readings for the summer months showed a very slight rise at both stations between 1920 and 1925; from 0.2 to 0.53 parts per million at Chillicothe, and from 0.2 to O.GT parts per million at Rome, to be particular. But when we examine the individttal readin.gs for 1920 and 1925 for the number of low points, we find at Chillicothe in 1920 one • See p. :jy;>. 422 Illinois Natural History Survey Bulletin reading under 0.3 parts per million out of a total of three ; while in L925 at the same station and approximately the same dates there were seven readings under 0.3 parts per million out of a total of twelve. At Rome the comparison was not quite so good, but there were two readings there under 0.3 parts per million out of a total of nine in the summer of 1925 (one of 0.2 and another of 0.1 parts per million), compared with one out of two at Rome in 1920. In the upper part of the Middle Lake, represented by Mossville, the average of nine readings June-September, 1925, was not far from one point lower than the average of five readings at Mossville in July- August in 1920, the exact average figures being 1.98 and 1.07 parts per million, respectively. In the lower part of the Middle Lake, represented by Al Fresco Park, the dissolved oxygen supply is subject normally throughout the summer months to great variations, due to rapid and extensive changes in the amount of phytoplankton present. But even here, the average of six mid-channel readings taken in the summer of 1925 (3.06 parts per million) was hardly visibly larger than the average of seven samples (3.01 parts per million) taken in approximately the same season in 1920. When we take into account the frequency of low readings, we find at Mossville only two readings under two parts per million out of a total of five in mid-channel in July-August, 1920, com- pared with seven under 2 parts per million out of a total of nine in 1925. At Al Fresco Park the number of readings in July-August, 1920, under 2.5 parts per million was three out of a total of seven samples taken in mid-channel; and in 1925, July-September, three out of a total of six. Table IX Changes in Bottom Dissolved Oxygen. Mid-channel, Chillicothe to near Foot OF Middle Peoria Lake, 1920 to 1925; Parts pim MrLLioN*t 1920 1923 1925 Upper Peoria Lake or vicinity: Chillicothe q 2^ ""ly-*"SO 0^54 (June-Aug.) 053I2 (June-Sept.) Rome 0.2^ (.July-Aug.) q 4^0 (June-Aug.) q gyS (June-Sept.) Middle Peoria Lake: Mossville i9g5 (July-Aug.) ^^36 (June-Aug.) ^^ (,,^9 (June-Sept.) Al Fresco Park 3 qi^ (July-Aug.) 4 gQ2 (June-Jul.v.) 3 Qg 6 (July-Sept.) * Exponent is number of readings averaged taken in the months indicated. t Water levels, all years, about normal for season. Visible increases in the bottom In Lower Peoria Lake and in dissolved oxygen in mid-channel the river between Wesley (opposite betzveen Copperas Creek Dam and South Peoria) and points shortly Beardstown after 1923. above Copperas Creek Dam the mid- summer dissolved oxygen readings since 1920 have generally been quite irregular and are best left out of the BoTTO-M Fau-XA, Illinois Rnt:K. 1913-1925 423 fliscussion at present. The first low. then high figures encountered, at various stations in this section of the river, sometimes reversed on the next round, are a consequence of the always varying mixture of fresh sewage received at Peoria and Pekin with more or less highly oxygenated water received from the frequently rich plankton-bearing wide-waters of the middle and lower sections of Peoria Lake. Omitting these figures, and dropping about 31 miles below the foot of Peoria Lake, to Liverpool, we find again for the -lO-mile section of river between Liverpool and Beardstown readings that are as a rule much more regular and certain in significance, but have relatively fewer of them for the summer months since and including l!i"20. than were taken in Peoria Lake. Taking Liver- pool, Havana, and Beardstown. 3L 40. and Tl miles below the foot of Peoria Lake, approximately, as representative stations for this section, we note little change upward, or actual decrease in the amounts of bottom oxygen jiresent between 1920 and 1923: averages or single samples taken at the three stations in July-September of those two years running: Liver- pool 1.75- in 1920* and 0.0' parts per million in 1923; Havana 2.16^ parts per million in 1920, and 1.1^ in 1923; and Beardstown 2.2' in 1920 and 2.4' in 1923. Between 1923 and 1925, however, an emphatic upturn in the bottom dissolved oxygen is indicated by our rather limited number of mid-channel readings from these same three stations. The comparisons with 1920 stood: Liverpool, increase from 1.75- to 4.45- parts per million; Havana from 2.1(i^ to 4.60'- parts per million; and Beardstown from 2.2' to 3.3' parts per million. It ap])ears likely, though not wholly certain, that the increases at Liverpool and Havana between 1923 and 1925 reflect improve- ment in methods of waste-disposal recently put into operation b\' the Corn Products Refining Company at Pekin ; and that the moderate relapse in the dissolved oxygen figures at some stations below Havana in the sum- mer of 1923, as in other recent years, is a conseepience of delayed fer- mentation of some of the carbohydrate wastes still received at Pekin qj Peoria. T.\BLE X. Changes ix Bottom Di.ssolved Oxyoex. Mid-chaxxel, LmcKPooL to Beards- Towx. 1920 to 1925; Pahts per JIiu.iox*t Sampling stations 1920 1923 1925 Liverpool ;^ 75I (.\ug.-Sept.) o.^UA^g.) 4452 (Julj-) Havana 2.16^ (Jul.v-Sept.) ^ ji (Jul.v) 4 g2 (Jul.v) Beardstown 2.2' <^'^''*' 2.4' '-^"^-^ 3.3 ' ^J"'*'' • Exponent is number of readings averaged. t Water levels, all years, about normal for season. 424 Illinois Natural History Survey Bulletin Uncertain index value of dissolved In the summer months of oxygen figures from the Peoria Lake 1920 and succeeding years dis- ivi'de-ivaters since 1920. solved oxygen readings from the wide-waters of Upper and Mid- dle Peoria Lake have agreed mainly in the single point of showing great irregularity. This is, however, not wholly unexpected, as both these sec- tions of expanded river lie in close proximity to the boundary line between the light and the heavy oxygen-producing plankton of the warm low-water period (in other words, the blue-green Algae and the chlorophyll-bearing Algae and Flagellata). The location of this line in 1920 and 1923 varied sometimes as much as the length of either lake in a few days or weeks, as there occurred shifts in water levels, temperature, wind, and sunlight. Under the most favorable conditions for the multiplication of the Chloro- phyceae and the green Flagellata, we occasionally obtained in 1920, even in the wide-waters of the Upper Lake, surface readings of dissolved oxy- gen topping 6 or 7 parts per million. A few days or weeks later, as the lower limit of the largely colorless or blue-green plankton moved several miles further downstream, it was not unusual to get bottom readings under one part per million more than three-quarters of a mile from mid- channel in the Upper Lake ; and readings under two parts per million at similar distances even in the lower part of the I^Iiddle Lake. Because of this unusually great variability in dissolved oxygen fig- ures, the minimum mid-summer readings are believed to be of more value than averages in estimating the fundamental or underlying sanitary con- dition in the Peoria Lake wide-waters. A short table of minimum dis- solved oxygen readings taken at long distances from the mid-channel line in the Upper and Middle Lakes is shown on p. 425. For comparison with the location of these minimum readings, the approximate full recent low-water widths of the lake on the side beyond the mid-channel line from which they were taken are also given. These show that at times both in the summer of 1920 and 1922 comparatively low bottom dissolved oxygen was not infrequent at stations well toward the margins of the east wide- waters of these two subdivisions of Peoria Lake. The 192-L- figures are of no value for comparison because of excessive dilution due to flood conditions all summer. The 1925 unpublished fig- ures by the State \\'ater Survey are apparently deficient in readings from stations at long distances from the center of the channel, in all sections of Peoria Lake. Although this is the case, it is believed it may safely be assumed that, as there was practically no change in the channel supply of bottom oxygen in the Upper and Middle Lakes between 1920 and 1925, so there cannot have been any appreciable amount of change of a permanent nature in the wide-waters, at least where open and compara- tively free from vegetation. During the summer season, our recent records of dissolved oxygen, both surface and bottom, in the east wide-waters of Lower Peoria Lake have in general been more consistently high in comparison with the indi- cations supplied by the bottom fauna than in either the Upper or Middle Bottom Fauna, Iixinois Rivek, 1913-1925 425 Lake. But this advantage is probably largely nullified at other seasons, as in the late autumn, when the first northwest winds carry over local pol- lution from the Peoria water front. Effects of this wind- and wave-borne pollution are apparent, in fact, on comparison of the number of small bot- tom animals with lowest tolerance taken in recent years in the Middle and Lower Lake. It is found, in brief, that if a few species that have survived in the Lower Lake since 1920 by virtue of unusual current of the very wide channel be excepted, the Middle Lake has recently yielded a larger number of small bottom species with low tolerance than has the Lower. Table XI. Bottom Di.ssoi.ved OxY(iE.\, Mid-si.mmer Low Point.s in Peoma Lake Wide-wateus, 1920 and 1922.* Reaches and stations Approximate full width of lake east of mid-channel line at recent summer low water levels miles Distance east of mid- channel line of low bottom dissolved oxygen readings miles Low readings of bottom dissolved oxygen p. p. m. Upper Peoria Lake Rome 1920 Rome 1920 Rome 1922 Rome, 21/2 mi. below 1920 Rome, 21/2 mi. below 1920 Middle Peoria Lake Mossville 1922 Mossville 1920 Mossville, % mi. below 1920 Al Fresco 1920 * Water levels, both year.s, about 0.87 426 Illinois Natural History Suhvey Bullktin species). .\11 of the non-current-loving Gastropoda except one (Cainpc- loiiia siibsoliduuij have been proved by their later records of extremely slow replacement to belong among the more sensitive of the small bottom species ; and the same is true of the single important burrowing mayfly nymph of the middle Illinois River (Hexagcnia bilincata). The most important of the several kinds of Sphacriidac (Musculium transvcrsum), on the other hand, has since 1920 shown itself, by its rapid recovery to and above 1913-l!)l.j figures in the worst polluted parts of the Peoria Lake channel and points above, to be unusually tolerant ; and stands as one of our best items of biological evidence that at some time between* 1915 and 1920 conditions on the bottom in those portions of the Illinois River were even worse than we found them in the last-named year. The declines in Gastropoda and Sphacriidac between 1913-1915 and 1920 were heaviest in the previously very rich section of river which ex- tends some nine or ten miles above the Spoon River bar at Havana. Here the drop in average numbers of Sphacriidac between 1915 and 1920 was from 1,709 to only 46 per square yard when all areas are combined; and the average decline in total Gastropoda was from 496 to only 20. In the three sections of Peoria Lake the declines in both of these groups in the same period were also very marked, but were actually small enough, in part as a consequence of previous damage suffered, so that they were largely or wholly made up in 1920 by the simultaneous increase of the pollutional worms and midge larvae. Other minor groups, in addition to the burrowing May-flies, that showed sharp declines in some or all reaches between 1915 and 1920 in- cluded the leeches, the caddis-flies (Hydropsychidac) . the Odonata. Plan- aria, and Ainphipoda, to name only the more important entries in a much larger list. Of these the leeches, the Odonata. the Plauaria, and Amphi- poda had enjoyed a wide distribution, in all reaches, (with the exception of the last two, usually in moderate numbers) previous to 1920. The Hydropsychidac had been confined principally to those sections of the river with hardest bottom and swiftest current in the 1913-1915 period. Increases in Tubificidac and The two groups of small bottom ani- Chironomidac 1915 to ig20. mals that showed large increases in num- bers in all sections of the river between LTpper Peoria Lake and Beardstown as pollution became heavier between 1915 and 1920 were the Tiibijicidac and the Chirouomidac. Both septic and pollutional Tubificidac of at least two genera and several species, in- cluding Tubifex tubifcx and several species of Liuinodrilus, always had been represented under the cleaner-water conditions of 1913-1915 and earlier, at least in moderate numbers in the muddier sections of the middle and lower Illinois River ; though they then attained large numbers in the sections of river here considered only in restricted localities subject to constant or occasional local pollution. The Chironouiidac before 1920 included more than a dozen species, several of which have since turned * See p. 439 and reference mentioned in footnote on same page. Bottom Fauxa, Iixixois Rher, 1913-1925 427 out to be of unusuall}' tolerant habit : and two or three which seem easily to stand pollutional conditions. ^lost important among the latter are : a variet}- of CJn'roiiomus pliiiiiosiis, with ventral blood gills of adaptable length for living under high or low oxygen concentration ; Chiroiioniiis lobifcnis; and a small form much like Chironoimis plumosiis that has been recorded from Illinois previously as Chironomis dccorus. The increases in both the Tuhificidac and the Chirouomidac between 1915 and 1920 were greatest in the three sections of Peoria Lake and in the formerly rich gastropod territory lying in the first ten miles above Havana : these comprising the principal reaches with practically con- tinuous soft black-silt bottom originally best adapted as habitats for these two groups as represented. The extreme average increase in Tuhificidac in numbers (in Upper Peoria Lake) was from 16 to 2.4().3 per square yard between 191-5 and 1920 : and in Cliiroiioiiiidac, in the same lake from 10 per square j-ard to 733. Below Peoria, the Tuhificidac at 46 per square yard, were about 30 times as numerous in the section between Liverpool and Havana as they were in the same area in 1915. Such an increase, however, is relatively small as compared with the more than 150- fold increase in Tuhificidac in the same time in L'pper Peoria Lake. In strong contrast with the rela- tively much greater increase of Tuhificidac than of Chiroiwntidac in the Upper Lake between 1915 and 1920, in the fonnerly rich gastropod terri- tory just above Havana the rate of increase of the Chirouomidac in the five years greatly exceeded that of the Tuhificidac. Here the Chirouomi- dac were multiplied more than 190-fold between 1915 and 1920, as com- pared with 30 times for the Tuhificidac. Coutinucd increase of the It was noted in 1922 that the increase Tuhificidac and Sphaeriidae in Tuhificidac. begun before 1920. had since 1920, and increase of gained momentum and that a verj- rapid leeches after 1924. rise in Sphacriidac was also under way in both of the two upper subdivisions of Peoria Lake. These gains were held, up to 1923, in both places, and be- tween 1924 and 1925 were extended in these or other reaches to still larger figures for both groups. Comparing the 1924 or 1925 figures with those of 1920 (or 1915) the most striking increases took place in Upper Peoria Lake, where these worms rose from 2.463 to 39.182 per square yard in four or five years ; in the Lower Lake, where the rise was from 78 to 6,919 per square yard : and in the section of the river from the Peoria and Pekin Union Railway bridge to Pekin, where there was an increase from none in 1915 (no collections in 1920) to 19,819 in 1925. Increases in the Tuhificidac in the Liverpool-to-Havana and the Havana- to-Beardstown sections between 1920 and 1925 were less, but in both of these sections the figures moved up from under 50 to around 2.000 per square yard. The increases in the Spliaeriidac from 1920 to 1925 were more irreg- ular than in the Tuhificidac, very possibly in part because this group is a 428 Illinois Natihal History Sih\ky Bi-ixetin preferred fish food to a much greater extent than the worms. Quite con- sistently, also, the Splmcriidac reached their greatest numbers in or near two of the best commercial fishing reaches ( Upper Peoria Lake and Liver- pool to Havana) in the continuous summer flood year 192-i (instead of 1935), when large numbers of the fishes were probably feeding in the "brush". In Upper Peoria Lake average numbers in all areas combined rose from 57 per square yard in 1!)2() to lS,ll-t in 192-1:; in the Middle Lake, from 3 in 1920 to 4,497 in 1925 ; and in the Lower Lake from 1.5 in 1920 to 1,122 in 1925. Just above Havana the gains approached those in Upper Peoria Lake—swinging from 46 per square yard in 1920 to 12,785 in 1924. Between Havana and Beardstown, in an always less favorable, because harder, bottom, the rise was only from 7 to 112 up to 1924, and to 2,561 in 1925. The leeches, after declining in numbers only moderately, from 1915 to 1920, as practically all the Illinois River species are of unusually toler- ant or even occasionally pollutional habit, rose hardly appreciably till 1924 or after. The largest part of their increase came between 1924 and 1925 and was most conspicuous in three short reaches : in Middle Peoria Lake, where they rose from none in 1920 to 548 per square yard in 1925 ; in the section between the foot of Peoria Lake and Pekin, where they rose from 58 in 1915 (no collections in 1920) to 1,511 in 1925; and in the ten miles immediately above Havana, where they increased from under 20 per square yard to more than 10,000 per square yard between 1920 and 1925. Below Havana increases were negligible in the five years following 1920. The marked increases, rather than decreases, noted in the total Tiibi- ficidac between 1920 and 1925 are in apparently significant agreement with the bottom dissolved-oxygen figures from the channel above Peoria in indicating little or no change in sanitary condition in the bottom muds since 1920. It should be kept clearly in mind, however, that the rich tubificid territory below Chillicothe has been referable to the pollutional, or sub-pollutional, and not the septic zone since that time. The leading member of the family in abundance there, also, has been a species of Limnodrilus ( Liinnodriliis hoffiucistcri), and not Tiibifcx tubife.v; though comparison of incomplete counts of samples from stations in the river above Chillicothe in 1923 with a few counts from Peoria Lake showed considerably larger ratios of Tubife.v tubife.v at the up-river, and so more heavily polluted, stations. The recent changes in abundance in an upward direction in and above Upper Peoria Lake both of Limnodrilus hoffnicisteri and the domi- nant sphaeriid, Musculium transvcrsum, which seems able to thrive almost equally with that worm in polluted bottom, may well be, for anything that we know to the contrary, illustrations of an unusual adaptability more or less recently and locally acquired. In other words, we may perhaps suppose that both of these two species found the strength of the pollu- tion over most of Peoria Lake rather too much for them at the crest of the pollutional wave that seems to have reached its height shortly be- Bottom Fauna, Illinois Rivkk, 1913-1925 429 fore* IIV^O; Init have since found conditions just about to their liking. So far as their sources of food are concerned, it cannot be behaved that there was any lack in that respect at any time in recent years : the Tithi- ficidac utilizing the bacteria and fine organic detritus of the bottom, and the Sphacnidac using a wide range of living or dead microorganisms, most of which are diatoms, unicellular algae, protozoa and the like, from the always rich plankton population overhead. The enormous increase in leeches in the section of river between Liverpool and Havana from 1!)"24 to 192.J, which seems to have attained the proportions of a veritable epidemic for whatever organisms they were living upon, occurred in a section where t)ne of their preferred food organ- isms (small bivalve Molliisca of the family Sf^liacriidac) was jiresent in very great numbers ; and as the leeches increased the Sf^luicriidac declined heavily. The more abundant species of leeches taken between Chillicothe and Havana from 1920 to 1925, as likewise before that time, have been five or six in number (HclobdcUa stagnalis, Hclobdclla ncphcloidca. Diiia microstoma. Erpobdclla punctata, and Glossiphoiiia complanata) , and are all known to feed upon snails, worms, various insect larvae, and other smaller bottom animals ; the first two and the fourth also being scavengers. The most abundant one of all in recent collections from the middle Illi- nois River, Hclobdclla stac/nalis. has been noted particularly by Dr. J. P. Moore as being partial to Sphacriidac . A special table showing the rise in leeches alongside the decline in Sphacriidac (principally Miisculiuiii transversum) in the 10-mile stretch above Havana, 1924 to 1925. follows. Tabi.k XII Inverse Changes in Abundanck of Leeiiies and Musculiutn transiH'rsvm. from 1924 TO 1925 IN THE Section of River Between Liverpool anii Havana; AVER.\GE Nr.MlSERS PER SlJlARE YARD Areas Year MuscuHii7n \ Total Iransversum : leeches Channel ( 1924 Extra-channel < i q,j; Channel and extra-channel areas combined. 1924 1925 430 Illinois Natural History Survky Bulletin feeding by carp in the early spring of 1935 ; or even possibly to unusually heavy mortality among the Sphacriidae as a result of overcrowding con- sequent upon their exceptionally rapid rate of increase between 1923 and 1934. The case, between the foot of Peoria Lake and Pekin, in which the leeches rose from only 5 to 34:1 pounds per acre while the Sphacriidae increased slightly, calls merely for the comment that if the leeches had not increased so much very probably the Sphacriidae might have gone much higher. Comparisons as between the years 1920 and 1925 also show a few instances where both leeches and Sphaeriidae rose simultan- eously over the five-year period, from more or less negligible to important numbers. Even if the correctness of the predator-victim relation be granted, however, it is reasonable to assume, up to a certain point, that the leeches might increase sharply as the species preyed upon increased ; the point at which increase gave way to decrease in the small bivalves de- pending upon the time of attainment of an unbalance in the multiplication rates of the two groups. Small or negligible increases. In the cleaner-water 1913-1915 1920 to 1925, in the Gastropoda period the Gastropoda had bulked large and other clcaiier-ivatcr groups, both in numbers and weight in all reaches of the river and Peoria Lake between Chillicothe and Beardstown. The group at that time included in all reaches not less than half a dozen species of unusually sensitive habit that were either generally abundant or abundant to common locally: Vivipara contectoidcs ; Vivipara siibpurpmrea; Lioplax suh- carinatus; Amnicola emarginata; Amnicola linwsa; and Souiatogyrus siib- globosus; all of which were practically wiped out in the deeper open water all the way from Chillicothe to Beardstown by the wave of pollution that reached its climax shortly before 1920. Two other important mem- bers of this group, Campcloma siibsoliduin and Pleurocera aciitum were also destroyed in great numbers, but much less completely so than the six first-named species ; Pleurocera, particularly, holding out in fairly good numbers in all portions of the Peoria Lake channel where there was un- usual current. The recovery in the total Gastropoda in the three sections of Peoria Lake between 1920 and ]925 was so small as to be practically unmeasur- able; the maximum rate of distribution (in the Middle Lake) being only 12 per square yard in 1925, consisting wholly of Campeloma subsolidum, which is unusually tolerant, and comparing with 49 to 130 per square yard of mixed species in 1915. Below Peoria, in the 10 miles between Havana and Liverpool, where the combined Gastropoda had averaged 496 per square yard in 1915, they rose in 1925 to a bit less than 40, from just half that number at the recorded low point five years before. The fact that more than half of the specimens taken in this section in 1925 were Vivipara contcctoides, however (the rest being Campeloma subsolidum) seems to reflect to some extent the results of the previously mentioned improvement in the bottom dissolved oxygen supply below Copperas Bottom Fav.na, Illinois Rivkr, 1913-1925 431 Creek Dam since 1923. Below Havana, to Beardstown, still greater va- riety was presented: two-thirds of the average consisting of three of the niore sensitive species (J'iz'ipara contcctoidcs ; Amnicola cmarginata ; and Atnnicola limosa) and only one-third of them being the tolerant Cam- pcloina siibsolidiiin. Here average numbers per square yard had been T9 in 1915; had dropped to 22 in 1920; and risen to 40 in 1925. Next after the Gastropoda the burrowing May flies of the genus Hcxagcnia were by all odds the most important of the cleaner-water spe- cies of the muddy reaches of the middle Illinois River in the 1913-1915 period and earlier. At that time they were taken as far north as Upper Peoria Lake, and were present practically everywhere between that sec- tion and the lower end of the river, though their habit of depositing their eggs in swarms, often quite widely separated, sometimes resulted in their appearing rather scatteringly in a fixed program of collections. In the summer of 1920 the common species, Hcxagcnia biliiicata, did not appear at all in collections above the foot of Matanzas Lake, 4 miles below Ha- vana : since that year it has been taken but a single time, and then in very small numbers, and very close to the edge, at one of the stations be- tween Peoria and Havana. Below Havana, as we would expect, the species has been taken a little more frequently in the last few j-ears : but did not occur at all in collections above Beardstown either in 1924 or 1925. Among the more important minor groups of the cleaner-water small bottom fauna of 1913-1915 that have shown slight if any recovery at all since 1920 may be noted: the Hydrof>sychidac and Odoiiata, among in- sects ; and among lower forms the Plaiiaria. \\'hile all of these groups except the Odoiiata occasionally attained quite large numbers around ten \-ears ago they did not either then or later amount to a great deal in aver- ages, and need not occupy time here for discussion. Colcoptcra, repre- sented almost solely in recent years in the deep open water by a species of Stciichiiis. have been principally confined to the vegetation near the margins both before and since 1920 ; the latter statement being also true of most of the Odonata. The principal Crustacea and Bryoaoa of the open water are more or less tolerant and have received mention in that connection in another place. Irregularity of abundance of tlic The larvae of midges, or Chiro- Chirononiidac since i()20- noniidae, have been noticeably irregu- lar in abundance in all of our recent collecting in the Illinois River. As illustrating some of the more im- portant changes, we found large decreases occurring between 1920 and 1925 in Upper Peoria Lake and in the ten miles above Havana, large increases in Middle and Lower Peoria Lake, and slight increases between Havana and Beardstown. Most of the species taken in the sections of the river covered since 1920 are pollutional or more than ordinarily tolerant; their natural food supply, of settled and bottom plankton and organic detritus, is almost everywhere abundant ; and there can be. therefore, hardly an)' question 432 Illinois Natural Hlstory Sukvky Bulletin concerning the general suitability of conditions. Because of the frequency of new broods, of the same or different species during the warm season, it is not to be supposed that the time of making collections would, in the long run, make much difference. This was verified in the case of the common Chironoinns phimosus in Middle Peoria Lake in 1922 and 1923, the average number of individuals per unit area of lake bottom not vary- ing seriously from one month to another between July and Sei^tember. The Chironoinidac also have shown themselves more susceptible than any of the other larger groups of small bottom animals to the effects of floods in the Illinois River in recent years : dropping sharply between 1923 and 1924 in nearly all reaches between Chillicothe and Beardstown, as a consequence, with hardly any doubt, of the unusually long continued and severe summer floods of the latter year ; and rising again sharply in the same areas between 1924 and 1925, as the river came back to normal warm season levels. Fuller details, with tables, of the changes in abundance of Chironoini- dac accompanying and following the 1924 high water are given on pp. 448-453, and tables showing the abundance of midge larvae in all reaches in a series of years will be found on pp. 463-465. Changes in Valuation as Fish Food and in Per Cent Composition by Weight Average poundage and composition In 1915, average valuations of total stocks, by reaches, in 1915. of the small bottom fauna be- tween Chillicothe and the La- grange Dam, about eleven miles below Beardstown, did not much ex- ceed 350 pounds per acre in any area of open water except the short reach of less than ten miles immediately above Havana, during the June- to-September season. Between Chillicothe and the Copperas Creek Dam the figures ran from around 170 to a little under 370 pounds per acre, and below Havana did not exceed 270 pounds. Quite in contrast with these records, the upper eight miles of the section between Copperas Creek Dam and Havana showed over 1,000 pounds per acre, and the lower nine miles, next above Havana, more than 2,700. At that time even the lower figures were not considered unexpectedly poor or un- favorable, in view of the generally rather hard or sandy bottom in which they were taken excepting in Peoria Lake ; and because in both the same and other sections of the river vastly richer areas were to be found in the connecting lakes and other backwaters. The comparatively low aver- ages obtained in the three sections of Peoria Lake have since appeared, so far as they arose from shortage of a considerable number of cleaner- water species, to reflect measurable injury there by pollution before 1915. The composition of the average total haul in 1915 in all the open- water reaches, including Peoria Lake, was made up always largely, and BoTTciii Fai'NA, Illinois Riveu, 1913-1925 433 frequently almost wholly, of some half-dozen species of large and small Gastropoda, all but one of which have since almost completely dis- appeared from the areas above the Co])peras Creek Dam. The average per cent by weight of the total haul made up by these Gastropoda in vari- ous reaches is shown in the table that follows, which includes laercentage showings as high as !)•") and only one lower than 60. In the Peoria Lake area rather less than a third of the totals of Gastropoda belonged to the unusually tolerant viviparid species, Caiiipcloina snbsolidiini ; and in the Liverpool-Havana section a not very different fraction ; pollution at that time not yet having affected seriously any of the more sensitive members of this grou]) of snails. In 1915, Spltacriidac occurred in numbers large enough to atfect valu- ations importantly in only three of the eight sections here recognized ; viz., in Upper Peoria Lake, where they made up 43 per cent of the total on the average; in the first section below Peoria Lake (Peoria and Pekin Union Railwav bridge to Pekin) where they contributed 31 per cent; and in the rich Liver]iool-to-Havana section, where the percentage was also 31. T.UiLE XIII Peecent.\ges by Weight Contributed to Totals of all Small Bottom Animals IN 1915 BY MOLLUSCA.t CHANNEL ANII EXTR.vCHANNEL AREAS COMBINED Reaches Av. lbs. per acre j Gastropoda per cent Sphaeriidae per cent Upper Peoria Lake 170 Middle Peoria Lake* Lower Peoria Lake 330 P. P. U. Bridge to Pekin 230 Pekin to Copperas Creek Dam 367 Copperas Creek Dam to Liverpool 1,064 Liverpool to Havana 2,757 Havana to Lagrange Dam 263 95 434 iLLTXois Natural History Survey Bulletin Both in the three subdivisions of Peoria Lake and in the first 20 miles below Havana, all of which areas had been onl\' moderate producers be- fore 1920, the reversal in composition of the fauna and the substitution of poUutional or unustially tolerant Tiibificidac. Chironomidac, and Sphaeriidae since 1915 for the Gastropoda and other cleaner-water species formerly dominant there, offered more striking and significant evidence of the increased pollution than the declines in poundage in the five years. Even the average poundage in the Lower Lake was cut down to about a third of the 1915 average, though the replacement of the cleaner-water by the pollutional and unusually tolerant forms served actually to increase the average poundage in Upper Peoria Lake and the upper portion of the Middle Lake in the five year interval of increasing pollution that ended in 1920. The outstanding decline in average poundage, accompanied by radical change in the composition of the small bottom fauna, occurred in the nine miles between Liverpool and Havana, where the high average of 2,T57 pounds per acre in 1915 gave way to an average of only 195 pounds five years later. The shifts in the composition of the fauna between 1915 and 1920 in the three sections of Peoria Lake reduced the Gastropoda almost 100 per cent, or practically to zero. The only surviving members of the group showing a trace in open water in 1920 belonged to the single un- usually tolerant stagnant-water form, Cauipcloiua siibsoUdnm, or, where there was unusual current, to the only slightly less tolerant current-loving Plcuroccra acntum. Here, as the Gastropoda fell, the combined pollu- tional and unusually tolerant Chironomidac and Tiibificidac rose to fig- ures, in terms of weight per acre, equal to 85 to 95 per cent of the aver- age total haul. In the sections of the river between Liverpool and Havana and between Havana and Beardstown the average poundage of Gastropoda was cut in the five years 80 to 98 per cent and the Chironomidac (prin- cipally the pollutional Cliironoiiiiis plnmosus) in the first-named section Table XIV Pbinclpal Changes in Valuation and Per Cent Composition by Weight of All Small Bottom Animals, 1915 to 1920. Channel anu Extr.\- Channel Areas Combined Reaches Average lbs. per acre Gastrop- oda per cent Bottom Faixa, Ii.lixois Rivkii. 1913-1925 435 made up 53 per cent of the average haul. The only surviving Gastropoda in the first nine miles above Havana belonged to one or the other of the two unusually tolerant species above mentioned ; and the more sensitive large Viv'iparidac and the smaller AmnicoUdac. which had been a promi- nent feature of this stretch of river in 1913-1915, were not taken at all. Below Havana the changes were less severe, though even here the Gas- tropoda dropped from 8(3 per cent of the average total haul to consider- ably less than half of it; while all Gastropoda that were found in 1920 belonged to the tolerant species of Plcuroccra above mentioned. Here also the Chironoiuidac, which had made up a negligible ixii'tion of the total poundage in 1913-1915, had multiplied until they contributed more than 30 per cent of it ; and the bulk of them belonged to the pollutional species, Chironouius pliiiiiosiis. Great increases in poundage in While in 1920 \aluation figures all reaches since 1920, due to higher on the average than in 1915 multiplication of the pollutional were obtained only in Upper Peoria or unusually tolerant (/roups. Lake, where the gains in the pollu- tional and unusually tolerant Tubi- ficidae. Chirononiidae, and Sphaeriidae had already outstripped the losses in Gastropoda and other cleaner-water species, it was clear by 1922-1923 that the continued rise of the single unusually tolerant sphaeriid, Muscu- liuiiL transz'crsnni, was rapidly lifting average iiomidages to formerly un- known levels also in Middle Peoria Lake and in the 9-mile stretch of for- merly very rich bottom just above Havana. The full extent of the change was not realized till the summer of 1924. At that time the returns from Upper Peoria Lake showed an average poundage more than twenty-five times that of 1920 and more than thirty-nine times that of 1915; the Middle Lake more than twenty-six times the average of 1920 ; and the Liverpool to Havana section almost twice the average poundage of 1915, and more than twenty-five times that of 1920. These great production averages, in 1921, of 6,737" pounds per acre in the Upper Lake, 1,110 pounds in the Aliddle Lake, and 4,996 pounds in the short section just above Havana, were almost wholly due to the rapid multiplication of un- usually tolei'ant Sphaeriidae (principally Musculiuin transversum), of which the contributions to the average total haul ran S(i.4 per cent, 92 per cent, and 94.3 per cent, respectively. Although there was a sharp decline in Sphaeriidae in L'pper Peoria Lake between 1924 and 1925 (still leaving the total poundage about six times the 1920 figures and more than nine times the 1915 figures), the rise in total poundage continued strongly ujiward after 1924 in the Middle Lake, going in that area from 1,100 pounds per acre to 2,014 pounds, due largely to increase in Sphaeriidae. It rose moderately also in the Liverpool-Havana section between 1924 and 1925, from 4,996 pounds to 5,355 pounds, in spite of a decline of large proportions in Mus- culiuui transz'crsuin. accom]5anied by a great increase in leeches known to be predatory with reference to many of the other small bottom species, 436 Illinois Natural History Survey Bulletin and of small bivalve Mollusca in particular. The decrease in Sphaeriidac in the Upper Lake between 1934 and 1925 was not accompanied by a cor- responding rise in the leeches ; and it may have been a consequence of temporarily increased feeding by fishes in that area, following a summer of almost continuous flood, which could easily bring large fish up the river in good numbers, but keep them occupied in the shallower, temporarily overflowed, or "brush", areas during its continuance. Perhaps the most remarkable change of all between 1924 and 1925 was the extension of the rapid gains in average total poundage to Lower Peoria Lake and to the previously comparatively poor sand-and-shell or lightly silted reaches between Wesley and Liverpool, and below Ha- vana. In the summer following 1924 the average poundage of the Lower Lake had risen from 230 to 1,009 pounds per acre, due to large increases in three of the pollutional or unusually tolerant groups, — Tiibificidac, Chironomidae and Sphaeriidac. In the two short sections between the foot of the Lower Lake and Copperas Creek Dam the rise in average weight was from less than 250 pounds per acre to the neighborhood of 1,300 ; in the first case this was due principally to an enormous increase in Tiibificidac; in the second, to still greater increase in the midge larvae. Between Havana and Beardstown in the same twelve months the average total haul was multiplied more than eight times, standing in 1925 at 1,135 pounds per acre, or almost ten times the average 1920 figure and almost five times the average of 1915. The increase between 1924 and 1925 below Havana was almost wholly in the unusually tolerant single kind of Sphaeriidac (Miisculium transt'crsum) that has recently been contributing so heavily to poundage figures in Peoria Lake, and that neither in the 1913-1915 period nor since 1920 until 1925 had contributed at all importantly to totals between Ha- vana and Beardstown. The very sudden appearance in 1925 of this small bivalve in such large numbers at stations in the first 31 miles below Ha- vana, where in 1913-1915 the greater part of the bottom was sand and shell, or otherwise harder than bottom usually selected by that species, Table XV Average Valuation, in Pounds per Acre, All Small Bottom Animals; Channel and Extra-Channel Areas Combined* Reaches 1925 169.7Upper Peoria Lake Middle Peoria Lake Lower Peoria Lake Foot of Peoria Lake to Pekin Pekin to Copperas Creek Dam Copperas Creek Dam to Liverpool 1,063.7 Liverpool to Havana 2,756.7 Havana to Beardstown 263.2 329.9 230.0 367.5 255.8 41.8 72.1 195.0 119.0 6,737.8 1,110.6 230.4 229.9 143.0 51.5 4,995.9 142.3 1,565.7 2,014.0 1,009.2 1,356.3 1,275.3 5,351.1 1,135.8 * The composition of these totals is shown in the tables on pp. 460-46S. Bottom Falxa. Illinois River, 1913-1925 437 can be explained only as in large part probably a special consequence of the unusual and long-continued summer flood of 19"24. It seems almost necessary to assume, in fact, that the flood not only carried down and deposited unusual amounts of rich sediment in certain parts of the Ha- vana-Beardstown section, but that it also actually moved good numbers of very young individuals of Miisciiliinn in the midsummer season of their greatest abundance, along with the sediment, many miles downstream. It is not. in fact, at all likely that a similar ett'ect. both in enrichment of the bottom deposits and in the multiplication of this particular species, could have been accomplished within the space of twelve months if the 19"2-1: flood had been confined to the usual short spring period. That has ordinarily been in recent years somewhere between the end of January and the end of April, when collections of MusctiliiiDi traiiszrrsuiii usually show practicallv all adults, which are nuich better able to hold their anchorage in soft bottom than the very young. In the section of this paper immediately following, the recent strong upward surge in abundance and valuation of the Sphacriidac is shown with a fair degree of probability to be a normal phase of a pollutional cycle or succession that evidently takes at times several years to complete itself. \'arious uneliniinable factors may also be concerned. In particu- lar, it should be pointed out that a portion of the recent very great pound- age increases in the polluted sections of river above Beardstown may reflect permanent reduction since 1913-11I1.5 in the population of bottom- ranging fishes, with consequently decreased inroads, for securing food, on the small bottom animal population. At least we can hardly assume that the increases have been wholly due in all areas to an abnormal multi- plication rate under the stimulus of a richer food supply, of its kind, that goes with a more polluted river. The comparison of commercial fisher- ies' census data of 1908 to 1914 with those of 1921 and 1922 to some extent bears out these strictures. Temporal Succession of the Leading Groups of Pollutional or Unusually Tolerant Forms in the Polluted Reaches Belovi^ Chillicothe Between 1920 and 1925 an accumulation of very suggestive evidence has come into our hands bearing on the order in which the more import- ant pollutional and unusually tolerant groups of the small bottom animals came into prominence as or after the more sensitive Gastropoda and other groups had disappeared from most of the reaches above Beardstown shortly after 191.5. For various reasons, however, comparisons based on the per cent composition of the average haul expressed in numbers per unit of bottom area have not been found as satisfactorv- for the purpose intended as those based on weight. On the basis of abundance, to take onlv a single example, the Tubificidac were shown to be the leading group 438 Illinois Natuual History Slrvey Bulletin in Upper Peoria Lake in 192(). with an average of 2,463 individuals per square yard, although the Chironomidac, with decidedly smaller numbers (only T33 per square yard, to be exact), contributed much more heavily to average poundage over the same area at the same time. Next, it is found that by that method of comparison the Tiibificidac continued to hold their dominance in that section of river continuously (except for 1921, when we made no collections) from 1920 to 1925, though in both 1924 and 1925 and possibly also in 1922 and 1923 the Sphacriidac had risen to the leading place, on a bulk and weight basis, having in fact a margin of around 300 per cent over the Tiibificidac in 1924. By scheduling the various groups on a basis of per cent composition by weight, on the other hand, we obtain a picture of the true "complexion" of the small bottom fauna rather than its population-distribution ex- pressed in confusingly different sized units, and see the main features of a succession that seems to have a real biological basis. Using this method of comparison, we find that the Chironomidac, represented very largely by the single pollutional species Cliiroiioinus pluiiiosus, led in all reaches between Chillicothe and Beardstown in 1920 where Gastropoda had fallen to less than 20 per cent of the average valuation total, this including the three sections of Peoria Lake, and the nine-mile section immediately above Havana ; and that Tiibificidac were following instead of leading in the two sections of Peoria Lake in which they were most abundant that year. In 1924, an essentially changed picture is presented ; with the Sphacriidac, in all subdivisions of Peoria Lake and part of the river reaches above Havana, holding the place that was held by the midge larvae in 1920; and with the Tiibificidac holding first place in two of the river reaches between Peoria and Havana and in the 31-mile section between Havana and Beardstown. In 1925, the Sphacriidac still held their lead in the three subdivisions of Peoria Lake and in the section next above Havana ; and the Tiibificidac were still first in weight between the foot of the Lower Lake and Pekin, though they had lost their lead to the Chironomidac between Pekin and the Copperas Creek Dam and to the Sphacriidac between Havana and Beardstown. It is evident that we have in the above data strong hints of some kind of a cycle. In order for this cycle to emerge into full light, how- ever, it is necessary to have in mind a few additional features of the biology not yet mentioned. The first of these is that, in very badly polluted muds in streams already infected with those worms farther up, Tiibificidac are ordinarily the first of the pollutional or unusually toler- ant groups to attain not only dominant numbers but dominant bulk and weight as well. This was well illustrated in work in the upper Illinois in 1911-1912, and more recently in studies on some less important water-courses in the State. The reason for it is not at all obscure, be- ing the simple fact, apparently, that the worms can float or roll into the new area; whereas the Chironomidac must ordinarily make their entrance from the outside, on the wing ; and the Sphacriidac, excepting Bottom Fain a, Illinois Rivkr. 1913-1925 439 those times (as in 1M54) when unusual summer floods seem to have moved their young and half grown, depend principally on the slow pro- cess of creeping over the bottom. If the above is a substantially correct view, the three sections of Peoria Lake and the ten-mile stretch of mud bottom next above Ha- vana are seen to have been very probably in the second (chironomid) stage instead of the first (tubificid) stage of the pollutional-biological cycle in the summer of 1920. This would ordinarily also mean that a short time before 1920 Tubificidac were probably the dominant group in bulk and weight in all or most of those areas. Though it is not neces- sary, in order for this to have been the case, (at least in Upper Peoria Lake) to assume that the pollution was greater in these parts of the river shortly before than during 1920. it is noted that the tendency of the data on the volume of business done by the Chicago Packers from 1916 to 1920 and of other evidence* is to suggest that it was at least for a time measurably more so. The third ( sphaeriid ) phase of the cycle had attained full swing in the three subdivisions of Peoria Lake and in some reaches farther down river by or before 1924. incomplete valuation figures for 1922 and 1923 suggesting that the Sphacriidac had attained the lead in point of weight in Upper Peoria Lake as early as 1922 or 1923. It is necessary' to point out also that dominance of the Sphacriidac (as represented almost wholly by the single species Miisciiliuiii transvrrsiiiii) in areas where Tubificidac and Chii-onuiinis pliiiiiosiis had previously held the lead, does not necessarily imply improvement in sanitary condition. That species, in fact, accompanied the Tubificidac as far up river as LaSalle in both 1924 and 1920 ; often attaining large numbers side by side with tens to hundreds of thousands of Tubificidac per square yard. The evidence from the bottom dissolved-oxygen readings, already presented, indicates also that there was no important change in the underlying sanitary condition above Copperas Creek Dam between 1920 and 1925. Evidence of a minor succession of Tubificidac-Chiroiioniidac- Sfiliacriidac in several reaches below Peoria Lake in the two years 1924- 1925 is also little less clearly implicit in the data. The sudden elevation of the Tubificidac to first place in weight between the foot of Peoria Lake and Pekin, between Pekin and Copperas Creek Dam. and between Havana and Beardstown in 1924 was without much doubt in great part the result of an involuntary migration downstream of the worms dur- ing the heavy and continuous summer floods of that year. It stands also as corroborative evidence of considerable value that putrescible sediment is carried during floods far past its normal resting place under more stable hydrographical conditions ; and goes far toward ex- plaining the lag not infrequently noted between the condition of the bot- tom sediments and the dissolved oxygen and plankton in the moving water overhead. It was very probably largely due to the temporary » Richard.son. R. E., BuH. 111. Xat. Hist. Survey, Vol. XV, Art. V. pp. 32S-332, Aug. 1925. 440 Illinois Natukal Histouy Sukvey Bulletin nature of this minor wave of pollution in the section of river between Havana and Beardstown that the Sphacriidac displaced the Titbificidac as dominants there by the next summer. In the short section of river between the foot of Peoria I^ake and Pekin, on the other hand, the Titbificidac still held the lead in 1935; and between Pekin and Copperas Creek Dam the cycle had progressed only to the second stage (that of Chironomidae) by the summer of that year. The succession above described is essentially temporal in character, representing the order of attainment of dominance of three groups of substantially equal tolerance, within the limits of the zone (mesosaprobic, or pollutional to sub-pollutional) between that of septic and cleaner- water organisms ; and in territory where no changes sufficiently great as to have altered zonal boundaries are known to have taken place be- tween the dates of its inception and completion. It has no essential connection with the broad zonal succession from septic to clean-water forms that occurs over a period of years in a circumscribed area as pollution gradually decreases ; or from the upper to the lower reaches of a stream septic at its upper end but long enough to permit fairly complete biological self-purification in the run to its mouth. While leeches came near exceeding either Tiibificidac, Chironomidae, or Sphacriidac in weight per unit of bottom area between Liverpool and Havana in 1925, their numbers and weight were as a rule irregular and unimportant in all other sections of river covered by the collections since 1920. They seem, so far as represented by the five or six com- moner species participating in the unusually large leech totals of 1925, to have no necessary connection with pollution, although apparently able to stand about as much of it on occasion as the other three groups named. Their sudden appearance within a limited area in very large numbers that year does not fit into the succession above described at any point, but may have had some connection with the floods of the summer of 1924 and the further fact that at least three of the commoner ones, including the most abundar.*; of all, Hclobdclla stagualis, are scavengers. Leeches have been observed recently in Rock River swimming near the edges in strong current after rapid rises of water level, and similar ob- servations have been made on the Illinois River. The long-continued summer floods of 192-1 may have given rise to considerable dispersal of leeches and at the same time seem to have favored an unusual rate of multiplication of the small bivalve Mollitsca (Sphacriidac) on which several of the leeches feed. Floods also wash in various dead animals, terrestrial and otherwise, which might add to the food supply of the scavengering kinds ; and unusual mortality among the Sphacriidac, fol- lowing the unusual multiplication rate and resultant crowding mentioned, may to some extent have served the same purpose. In the following table illustrating succession, based upon average weight figures in pounds per acre, the small numbers at upper right of group names represent the percentages of average total hauls contributed by the groups ; groups with percentages under 5 being ignored. The Bottom F.m x a, Ii.i.ixois Rivkr, 1913-1925 441 larger figures at left preceded by letter A are the phase numbers of the grotips constittning the long pollutional cycle that started shortly be- fore ];)20: phase number 1 (Tiibificidac dominant) having been missed, and apparently falling somewhere between IDIT and 1i>v;i). The large figures at left preceded by letter B are the phase numbers of the minor and apparently temporary pollutional cycle that started in certain reaches below Peoria Lake during the severe summer floods of 1924. Table XVI Table Showing SrccEssiox of Leading Groups of Small Bottom Animals, Based ox Peroextage Composition dy Weight of the Average Hall; 1915 to 1925 Exponents are percentages of the average total poundages per acre contribtited t)y the leading groups of small bottom animals 1915 Upper Peoria Lake Gastropoda''^ Sphaeriidae^ Middle Peoria Laket Lower Peoria Lake Gastroi)Oda°' P. P. U. R. R. Bridge to Pekiu Gastropoda?- Sphaeriid^e'' Pekin to Copperas Creek Dam Gastropoda"' Sphaeriidae" Copperas Creek Dam to one mile above Liverpool Gastropoda"' Sphaeriidae" One mile above Liverpool to Havana . Gast/'opoda'^ Sphaeriidaef" Havana to Lagrange Dam Gastropoda" Sphaeriidae'- 1920 Upper Peoria Lake A2 Chironomidae^ Tubifieidae" Middle Peoria Lake A2 Chii-onomidae'' Tubificidae^^ Lower Peoria Lake A2 Chironomidae^ P. P. U. R. R. Bridge to Pekin no collections Pekin to Copperas Creek Dam no collections Copperas Creek Dam to one mile above Liver- pool no collections One mile above Liverpool to Havana A2 Chironomidae^ Gastropoda'"^ Leeches'* Havana to Beardstown. . . Gastropoda''* Chironomidae^ Leeches^' •Holdover from 1913-1915. t CoUettinns from upper third of Middle Peoria Lake included with Upper Lake in this part of the table. 442 Illinois Natural History Survey Bullktin Table XVI—Concluded. Upper Peoria Lake A3 Midrlle Peoria Lake A3 Lower Peoria Lake A3 P. P. U. R. R. Bridge to Pekin Bl Pekin to Copperas Creek Dam Bl Copperas Creek Dam to one mile above Liverpool A3 One mile above Liverpool to Havana A3 Havana to Beardstown Bl 1924 Uphaeriidae"' Tuhificidae^" Uphaeriidaef'- Sphaeriidae" Tubificidae'-' Tuiificidae*" Tuiificidae" Sphaeriidae"^ Sphaei-iida^' Tubificidae'^ Leeches" SpUaeriidacf- SiJhaeriidae" Leeches^^ TuMficidae^ Sphaeriidae" Gastropoda''* 1925 Upper Peoria Lake A3 Sphaeriidae'' Tubificidae'' Chironomidae^" Middle Peoria Lake A3 Sphaeriidae''^ Chironomidaef Leeches'* Lower Peoria Lake A3 Sphaeriidae'^ Chironomidaef Tubificidae^ P. P. U. R. R. Bridge to Pekin. Bl Tubifiddae'^ Leeches^° Sphaei-iidae^^ Pekin to Copperas Creek Dam B2 Chironomidae^' Leeches' Copperas Creek Dam to one mile above Liverpool. No collections One mile above Liverpool to Ha- vana A3 Sphaei-iidae*''y Leeches" Havana to Beards- town B3 Sphaeriidae" Tubificidae' Gastropoda'* t The leeches are apparently preying upon the Sphaeriidae, which are just abdicating- first place, which they held over from 1924. • Largely holdover from 1913-1915. Competitive Relations Among the Small Bottom Animals Among the three groups of small bottom invertebrates that have figured largest in abundance and valuation in the muddy reaches of the Illinois River above Havana since 1920 (Tubificidac, Sphaeriidae, and Chironomidae), it cannot be said that competition for food has been at any time an important influence on numbers. Insofar as these three groups use the same food, namely, the living bottom plankton and bac- teria and the settled plankton and fine organic detritus, they are com- petitors in a general sense. But by reason of the jjractically limitless amounts of those materials available, at least in the reaches of the Illi- nois River with less current, competition for food is probably non-ex- istent in actual practice, although all three groups have often in recent years been found associated in large numbers over the same bottom area. Bottom Faixa, Illinois River, 1913-1925 443 The gastropod Molliisca. embracing several species of large snails capable of ingesting objects as large or larger than the eggs of many insects, and having the habit of taking in practically everything that lies in the path of their tongues as they crawl over the incriisted surface of sticks, or stones, or dead or living shells, have bulked so small in the Illinois River bottom fauna above Beardstown in recent years that depre- dation by them on egg or very immature stages of other organisms may be regarded as little if any more than negligible. In years previous to 1920, on the other hand, the fact that they once became dominant in an area seems to have given them a more or less permanent and, if abundant, often almost undisputed foothtild there. This was with little doubt in some part due to the inability of other bottom invertebrates, which plaster minute eggs on solid objects on the bottom, to multiply in great numbers where the rasping tongues of hundreds or thousands of Campcloina and Vivipara per square yard were in daily operation. The hold of the larger Gastropoda on the territory once occupied by them in great numbers was doubtless also strengthened against some other- wise likely associates, by the smothering blanket of slime spread over and binding together the fine bottom ooze and shutting off the air over large areas from eggs or ver}- young stages of other sjiecies living therein ; and against still others (as possibly Tiibificidac and young Chiroiioiiiidac ) by the sheer bulk, weight, and slow motion of these snails, which could easily result in smothering many small organisms of other sjiecies un- fortunate enough to be in their way. Thus it is no surprise to find that our older data, obtained when large Gastropoda led in abundance in many reaches of the Illinois River, strongly suggest that great numbers of these snails act as a bar against the increase in large numbers in their inmiedite neighborhood of either Cltironomidac. Sphacriidac, or Titbificidac. Only those Tuhificidac of less poUutional habit are here referred to, Tubifcx tuhifcx being unlikely, because of its more distinct jireference for very foul bottom, to reach great numbers in muds inhabitable by the more sensitive J'iviparidac even if the large snails were not there. Of the larger species of Sphacriidac it was noted in years prior to 1980 that Miisciiliimi transrcrsiiiu was frequently comparativelv abundant along with considerable numbers of large I'iriparidac. thougli not likely to be present at all where riviparidac reached maximum num- bers. The indicated greater immunity to both direct and indirect injury by the large snails exhibited by this small bivalve, than by Cliironoiiiidac and Tuhificidac, quite possibly had some connection with* the fact that they are born alive with a fully formed shell, and of a size rather larger than that of food objects usually taken by the large Gastropoda. Leeches, concerning whose depredations on Sphacriidac mention has been made in a preceding section, are also known to prey upon Tuhificidac, but we find no positive evidence of that in our recent bottom fauna data. In the stretch of river between Liverpool and Havana the Tuhificidac in fact increased about "20 per cent between 19-,;4 and 1925, 444 Illinois Nati'Ral History Si'RVky Bulletin as the small bivalves, thought to be the object of leech depredation, de- clined. Other strictly predaceous bottom species that are known to af- fect the abundance of the less well protected small bottom organisms in many waters, such as certain larval Colcoptera and Odonata, have not been represented in important numbers in recent Illinois River and Peoria Lake collections from the deeper open-water areas. Accessibility, Quality, and Extent of Use as Fish Food Probably quite as effective as competition among themselves in keeping down numbers of some groups of the small bottom animals is their use as food by the more common bottom-feeding fishes. Also there is not lacking circumstantial evidence that neglect of some groups by the fishes permits them at times to increase while the stocks of others are being drawn down. The evidence from Professor Forbes studies on the food of the buffalo, red-horse, and carp-suckers from scattered localities in Illinois ; those of Cole on German carp in Lake Erie ; and those of the State Natural History Survey in the past three years on the feeding habits of carp, buffalo, and other sucker-mouthed fishes of Rock River, strongly suggests that, in general, these fishes prefer small bivalve mollusca (Sphacriidac) , various kinds of larval midges (Chironomidac) , the bur- rowing May-flies (Ephcuicrinac } . and the larval caddis-flies (Hydropsy- cliidac) to Twhificidac, leeches, and large Gastropoda. The evidence as re- spects avoidance of the Titbificidac by the large bottom-ranging fishes is of course not conclusive at present, consisting in their almost total ab- sence from more than a thousand stomachs examined (from Illinois waters only), although the worms are known in a good portion of the cases to be present in at least moderate numbers in the areas from which the fishes came. Several possible reasons for this seeming neglect of the Tubificidae by the principal large bottom-feeding fishes suggest themselves. For one thing, the Tubificidae are generally quite small, and are accustomed to withdraw themselves instantaneously into their tube-like burrows, ex- cavated in the bottom nuid, at the least disturbance. This habit may easily afford effective protection to them when their numbers are moder- ate and when they are interspersed between larger, more accessible, and more acceptable small bottom kinds, as is apparently most frequently the case where much feeding by the large bottom-ranging commercial fishes goes on. When they are extremely abundant, on the other hand, the dis- solved-oxygen supply is likely to be so low that little feeding by the fishes can be believed to take place during the active warm season in their territory. For the discussion of data that suggest, on the other hand, that the large bottom-ranging fishes do occasionally under unusual con- ditions, and perhaps more or less accidentally, eat large quantities of Tubificidae, see page 450. I Bottom Pat.xa, Illinois River, 1913-1925 445 The complete exclusion of the Tiibificidac from the dietan,- of the bottom- feeding fishes, even if it were to occur where they are most abun- dant, would cut down the total stocks of small bottom animals available, measured in pounds per acre, much less than the exclusion, under recent conditions, of the Sphacriidac ; but. on the average, apparently, not much if any less than the exclusion of the Chironomhiac. Thus, in Upper Pe- oria Lake in 1924. the Tiibificidac amounted on the average to approxi- mately 1.300 pounds out of a total of all kinds of small bottom animals of somewhat more than G.OOO pounds per acre; while the Sphacriidac averaged well over 5.000 pounds. Their occurrence in average poinid- ages over .500 to the acre during the years 1924 and 1923, was confined to three sections of the Illinois River, all above Pekin. In most of the few cases since 192((, in fact, where the ])ercentage of the total weight of all small bottom animals made up of Tiibificidac was high, the total weight of all kinds was low. and the average weight of the worms was considerably under 200 pounds per acre. In rather marked contrast to the Tiibificidac. the four more important of the apparently preferred groups in Illinois streams, the Sphacriidac. the Chironoinidac. the Ephcmcridac and the Hydropsycliidac . are all visibly larger; and they are accustomed, even when burrowers (as are some of the Chironoinidac and all of the formerly common Ephcmcridac of the Illinois River), to remain for considerable times outside of their burrows. Even the larvae of the more abundant caddis flies of the Illinois and Rock Rivers (the Hydropsychidac) live a free life upon the bottom much of the time, though they show a preference for cracks or crevices in rock or other hard bottom when it is available, and withdraw into a hard case made of sand grains just before pupation. In spite of the rather large degree of protection afforded theni by such habits they are not a small element in the food of several of the larger bottom-feeding fishes of these rivers. Because in the season of 192.5 the total weight of the leeches in the reach Liverpool to Havana rose to the unprecedented figure of over 2.500 pounds per acre, it is of interest, next, to inquire whether this rather large supply of potential animal food was likely to be of any important actual value to the large commercial fishes that range the bottom of the open Illinois River where the leeches were found. Such answer as is af- forded by the study of data from a variety of sources is not in favor of placing a high value on the leech crop as fish food. It is true that Pro- fessor Forbes* more than forty years ago found leeches in the stomachs of 3 out of 113 channel cat examined from Illinois waters, and also in the stomachs of (i out of 49 bullheads ; and large leeches are reported as being used occasionally as bait for channel cat in Rock River. But chan- nel cat and bullheads made up then as now only a rather small percentage of the total fish catch, whereas the more important group, from the point of view of bulk and weight and numbers, including the bottom-feeding • Various papers on the food relations of fresh-water fishes, published between 1877 and 1SS8 by this laboratory. 446 Illinois Natural History Sikvf.y Bullktin buffalo, red-horse, and suckers, furnished only a single specimen which had eaten a leech out of a total of lol specimens examined. Colej, in study- ing the food of carp in Lake Erie (1903) found no leeches at all in 33 specimens examined, but did find an abundance of small bivalve Mollusca and Chironomidac, much vegetation, and other minor materials. In our examination of the food of the larger bottom-feeding fishes of Rock River, not completed, 92 specimens of carp have shown no leeches taken as food ; 109 buffalo, of three species, showed only one leech eaten (by a mongrel buffalo); 116 suckers and red-horse and silver carp of various species showed no leeches eaten at all ; and over 900 channel cat and 40 other catfishes of various species showed no leeches eaten at all. Leeches evi- dently play a more important part in the food of shore fishes, particularly those that feed among vegetation—including sunfishes, perch, black bass, some perch-like fishes, sculpins, and bullheads, as is shown by PearseJ in his studies (1915-1916) on the food of fishes in some of the inland Wisconsin Lakes. The younger and thinner-shelled specimens of the large gastropod Mollusca are known to be used as food by the large species of red-horse, but did not appear in the food of buffalo, suckers, or silver carp examined by Professor Forbes from Illinois Waters. Neither have they been found by us in any of the specimens (upwards of 300) of carp, buffalo, red- horse, etc., examined in the last three years from Rock River. These snails, it is true, are comparatively rare in most of the Rock River. But in those reaches of the Rock River about and above Rock ford, where large Viviparidac are common, while they do not appear in the buffalo, suckers, red-horse or carp stomachs, they are found frequently in the stomachs of channel cat, the only fish we at present know of which seems capable of handling them. Professor Forbes called attention to this nearly 40 years ago, and today we find, as he did then, that the large channel cat are able to withdraw the bodies of full-grown heavy-shelled specimens of Campcloma and Plcurocera from the shells and swallow them with no hard parts attached except sometimes the operculum. The value of these larger snails as fish food thus appears to be quite limited, much as that of the several kinds of common leeches ; both entering into the food, for the most part, of a single minor group of the large bottom-ranging fishes (the catfishes), and being avoided by the more important, carp, buffalo, and allied kinds. From the point of view purely of accessibility, then, of the more important constituents of the small bottom fauna as fish food, the destruction of the large Gastropoda in the Illinois River by pollution since 1915, and their replacement by still larger numbers and greater total bulk of the more acceptable and more accessible small bivalve Mollusca (Sphacriidac) . may be regarded as a benefit rather than an injury to the commercial fishery, at least in those areas where the pollution which ac- t Cole, L. T., The German Carp in the United States. Kept. U. S. Bureau of Fisheries, year ending June 30, 1904, Appendix, pp. 523-641. t Pearse, A. S., The Food of the Shore Fishes of Certain Wisconsin Lal^es Bull. U. S. Bureau of Fisheries, Vol. XXXV, 1915-1916, pp. 247-291. Bottom Fauna, Illinois River. 1913-1925 447 complished the change does not hold the oxygen down to or below the danger point for the fishes during the active feeding season. On the score of quahty—inchiding both healthfiihiess and suitabihty for producing edible quality in fishes which consume it—the recent "rich" food supply in the more polluted sections of the Illinois River both above and for some distance below Peoria seems open to indictment on more than one count. While "gassy" fish have been complained of at various places along the Illinois River for many years, these complaints have be- come much more insistent in the Peoria Lake region since ]920. Since that time eastern receivers have been accustomed to make price discrimina- tions against Illinois River fish, more particularly, carp, for that reason. The local complaints have agreed that the "gassy" taste is more pro- nounced in winter, when the river and lakes are frozen over, without, however, advancing a satisfactory e.xplanation why that should be so ; the putrefactive processes in the bottom sludges being clearly much less active and productive of nuisance under winter temperatures than during the warm season. On reflection it is seen that a very simple and purely physical theory satisfactorily accounts both for the central fact of "gassi- ness" and for its stronger expression under winter conditions. Direct absorption by the tissues of odors from some varieties of food eaten is well known in man. The possibilities in fishes, such as carp, which, if they do not actually take up by choice appreciable quantities of foul- smelling sludge, do at times feed heavily upon small bottom organisms that have done so, need only to be mentioned to be realized. In the fouler areas ranged over by carp, not only the small oligochaetes, which swallow mud or sludge in large quantities, but also the small bivalves, certain snails, and other bottom species that take bottom detritus accidentally along with the bottom plankton or incrusting plant and animal growths, are likely to develop strong odors, which in turn will be reflected later on in the odor and taste of the fishes. During the summer season, of course, decompositions in the muds are likely to be much nearer their end points than in winter ; and the exhalation of such odors through blood- stream and skin is doubtless at a relatively high rate, corresponding with the degree of activity of the fish. Another point of possible importance is the ordinary habit of the larger commercial fishes of feeding much farther away from the channel in spring and summer than in late fall and early winter. Fishes, on the other hand, which continue to feed a a short time after the bottom is chilled, are likely to take in directly or indirectly material capable of producing especially otifensive odors, as its temperature is raised to that of the body of the late feeder. And those fishes which go into winter dormancy immediatelv after a heavy feeding on chilled bottom will retain such odors as are absorbed more fully and longer because of the lessened exhalation rate that accompanies inactivity. Among the carp taken since 1920 at and above Peoria, also, a very high rate of disease has been observed; the percentages of affected fish to totals recently examined going above 50 at most stations. There are 448 Illinois Nati'ral History Survky Bulletin some grounds for believing' that the most prevalent of the diseases noted may be connected with the character of the food supply. As the carp is the only surviving commercial kind that is now able to maintain fair numbers in these polluted areas, comprising probably more than nine- tenths recently of all fishes taken, it can be seen that a large part of the entire local fishery is atfected. Dr. David H. Thompson was engaged for some time in a special study of the diseased carp in the middle Illi- nois River, and his report on this subject is published in this bulletin.* Changes in Abundance of the Principal Groups During and Just After the Continuous Summer Floods of 1924 Of the six summer seasons, 1920 to 1925, all except that of 192-1 had the crest of the spring freshet not later than the middle of May, with gage height at Peoria dropping to between 11 and 9 feet before the end of June, and continuing within a range of 8 to 10 feet through July, August, and September. The warm season of 1921 was characterized by continuous summer floods, with the Peoria gage ranging 11 to 18 feet in June, 18 to 19 feet in July, and 19 to 20 in September. What appear to be direct effects of these ver}' unusual flood conditions are seen in the figures of abundance of midge larvae for the season of 1921, which al- most uniformly showed decreases, some of them quite large, over the figures of the year of more normal summer rainfall preceding. The larg- est decreases were shown in the three reaches (Middle and Lower Peoria Lake, and foot of Peoria Lake to Pekin) where Chironomidac had been most abundant the year before. The single section, in fact, of those ex- amined, in which average numbers of midges went above the figures of 1923 was that between Havana and Beardstown, where CItironoiiiidac had been comparatively rare in previous collecting seasons, and into which it is possible that the flood of 1924 introduced them in somewhat more than usual numbers. Evidence tending further to confirm our supposition that the de- creases in the Chironomidac in 1921 were due to the washing away of successive broods in the egg or early larval stage, is furnished by the data of abundance of the larvae in the summer season of 1925. Under the normal summer gages of that year large increases in midge larvae were shown in almost all reaches, with the largest increases falling mainly within the territory of largest decreases in the year preceding, if a single case of very unusual increase, in the short reach between Pekin and Cop- peras Creek Dam, be excepted. The latter quite possibly reflected the re- sult of some unusual attraction furnished temporarily by the odors from the Peoria and Pekin wastes. * Vol. XVII, Art. VIII, The "knothead" carp of the Illinois River. Bottom Favxa, Iixixois Rivkr, 1913-1925 Table XVII 449 CllAXGKS* IN AlirXDAXCF. OF ToTAL CuiROXOMlnAE DrRIXG A.ND iMMF.niATELT PoLLOwrxG THE CoNTixrors Sr^iMER Floods of 1924: Average Numbers per Square Yard Reaches and subdivisions 1923 to 1924 1924 to 1925 Channel Extra-channel Channel Extra-channel Upper Peoria Lake. .. —57 Middle Peoria Lake.. —557 Lower Peoria Lake... —1,059 Foot of Peoria Lake to Pekin —400 Pekin to Copperas Creek Dam —104 Copperas Creek Dam to Liverpool —62 Liverpool to Havana.. —36 Havana to Beardstown. +4 * Increases are represented by the —145 —383 —117 450 Illinois Natural History Sukvky Billetin of field operations in the region of Havana before 1920. It is evident that changes in abundance of the small bottom animals, provided they are of kinds not so easily washed away as the majority of the commoner Chironomidae of the Illinois River seem to be, would be in some measure during a growing season of continuous flood in proportion to the extent of this migration. Evidence confirmatory to some extent of this view of the case seems to be furnished in the fact that the Sphacriidac showed the largest in- creases during the flood of June to September, 1924, in Upper Peoria Lake and from Uverpool to Havana, where, in each case, that group had been most abundant the year before, and where, in the case of the first area mentioned, there is a very large acreage of "brush" territory con- tiguous to the area represented by our collecting stations. Likewise, the large decreases in the Sphacriidac between 1924 and 1925, along with the presumable re-entrance of the fishes into the deeper and more open waters for spring and sunnner feeding, also took place in the same two sections of river (i. e.. Upper Peoria Lake and Liverpool to Havana), where the greatest increases had occurred the year be- fore. This finding is, in its turn, at least consistent with the popularly held supposition that large fishes which move about considerably are alert to discover and utilize the richest feeding grounds, provided of course, that the oxygen supply is adequate. In order for the latter to have been the case in Upper Peoria Lake in 1925. it seems necessary to assume that the bulk of the cutting down of the surplus stores of Sphacriidac laid up there in 1924 was done from April to June rather than in July and August. Table XVIII Changes* in Abundanct? of Musculium Transversiini During and Immediatet-y Following the Continuous Summer Floods of 1924; Average Numbers per Square Yard Reaches and subdivisions 1923 to 1924 Channel Extra-channel 1924 to 1925 Channel Extra-channel Upper Peoria Lake... -|-16,977 -f-14,533 —15,242 —14,828 Middle Peoria Lake.. -1-1,844 -f-1,044 4-3,164 4-1,596 Lower Peoria Lake... 4-130 -|-49 4-1,238 4-327 Foot of Peoria Lake to Pekin 4-10 -|-132 -|-1,064 4-36 Pekin to Copperas Creek Dam —82 -^124 -^34 4-38 Copperas Creek Dam no collect- no collect- no collect- to Liverpool —26 ions 1923 ions 1925 ions 1925 Liverpool to Havana. 4-17,928 -f7,200 —5.424 -7,058 Havana to Beardstown —199 -f-40 -|-5,290 -+-1,843 * Increases are represented by the -j- sign, decreases by the — sign. BoTTo.M Fauna. Illinois River, 1913-1925 451 Just why it was that the Sphaeriidac increased rather than dedined in most of the other reaches, between the ^liddle Lake and Beards- town (except the LiverpooI-to-Havana reach) in 1925 is not wholly clear. The figures stand about as they would, however, if two assump- tions are granted : first, that an excessive rather than merely normal amount of feeding by the fishes is likely to occur where the bottom fauna is richest, with the limitations above noted; and, second, that the normal rate of increase in the SpliOi'riida^ in recent years, both in the areas of its greatest abundance and elsewhere, may have been in excess of normal demands from the present fish population. Our data on abundance of Tiihipcidac from 1023 to 1925 in the reaches between Chillicothe and Beardstown, if the single instance of Upper Peoria Lake be excepted, reflect an irregularity that can be as- cribed only to the operation or effects of several influences. In the first place, these worms increased greatly, both in the channel and extra- channel zones in L'pper Peoria Lake, during the heavy floods of the summer of 1924. and decreased in the same ])art of Peoria Lake in 1925; in both instances in quite a similar way to the changes of the Sfhociii- dac. In this part of Peoria Lake, the stronghold of both the S/^hacriidae and Titbificidac between Chillicothe and Beardstown since 1920, and where, in fact, the worms reach vast numbers in close juxtaposition on the lake bottom with the small bivalves, it is probable that for several years the large bottom- feeding fishes, when they have fed there at all. have been forced to take large quantities of the small worms as food in order to get the Spharriidac. This being so, the explanation of the rise T.\HLE XIX Changes* ix Abundance of Tot.vl TuBrFiciD.\E During and Immediately FoixowiNG THE Continuous Summer Floods of 1924; Average Numbers per Square Yard Reaches and 452 Illinois Nati-ral History Survey Bulletin in numbers of the worms under the flood conditions of 1924, and the sharp decHne in the much more settled summer following, is probably largely the same as that offered above for the changes in the common small bivalve, Miiscjiliuin transz'crsum. The unusually larger increases in Tiihificidac in 1920, following the flood year 1981, in several of the reaches below Upper Peoria Lake, and including even Havana to Beardstown, seem to call for a less simple explanation. In the first place, in none of the reaches of river below Upper Peoria Lake could it be said that the Tiibificidac and the Sphaerii- dac were so crowded together on the bottom in the years before 1925 that the carp and buffalo would be compelled to take them in order to get the ordinarily preferred Sphacriidac ; that is, the Sphacriidac could be more readily foraged for separately in these areas than in L^pper Peoria Lake, and the Tiibificidac to the same extent left alone. Again, as has been mentioned in a preceding section, there are strong reasons for supposing that the unusual all-summer floods of 1924 brought down "seeding-stock" of the worms themselves, along with a rich supply of organic sediment, into several reaches of river between Peoria and Beardstown that had been poor bottom fauna and fish producers for many years before because of prevailingly hard sand-and-shell or lightly silted clay bottom. The sharp spurt upward of the Tiibificidac in the extra-channel areas of the reach between the foot of Peoria Lake and Pekin in 1925 seems most probably to have resulted from a concentration of local pollution on the east side of the river at Pekin. The richest hauls of Tiibificidac, which account largely for the high average, were taken on the Pekin side, above the Corn Products Refining Company's plant, and might have originated either at Pekin or at South Peoria. We may now return again to the Chironoiiiidac to inquire what may have caused that group, previously listed with evidently good rea- son among the preferred foods of several of the most important of the bottom-feeding fishes, to increase its numbers, either sizably or heavily in all reaches above Copperas Creek Dam in 1925, while the Sphacriidac declined sharply in Upper Peoria Lake and showed relatively small or negligible increases in all the other reaches in the same distance. The answer is probably to be found partly in the fact that, in the case of the Chironoiiiidac, we are dealing with a larger number of kinds and a much wider distribution of broods over the growing season ; whereas close to 99 per cent of the Sphacriidac in many sections of the river has recently been made up of Musculimn transvcrsum, a species whose single heavy bearing season has in recent years in the Illinois River usually come as late as Jidy or August. Even more important, per- haps, is the fact that the edible-sized larval Chironomidac periodically leave the river, actually for two weeks or more between emergence and egg-laying, and effectively for longer periods ; while the small bivalves are permanent residents on the river floor. As a consequence, largely of one Borro.M Faina, Illinois Rivkk, 1913-1925 453 or the other of these considerations, the large schools of carp and buffalo that made their way upstream in the early spring of 1925 could con- ceivably have made heavy inroads into the stocks of gravid Sphncriidae at a point of especial vulnerability, antedating the normal time of heaviest birth rate by several weeks. The same schools of bottom-feeding fishes, however, could have passed up the river easily between dates of pupa- tion and emergence of some of the earlier broods of the Chironoinidac and so have had relatively little effect on abundance of those particular kinds through the rest of the same summer. Or, in an unusually late spring, the same effect, in greater or less degree, could have been i)ro- duced several weeks later in the season. Comparative Abundance in the Channel and Extra-channel Areas, 1920 to 1925 Comparisons of channel and e.xtra-channel figures of abundance of the Tiiblficidac for all collecting years 1!)2() to 1925 shows that in all the reaches above and below Peoria Lake, and in that part of Peoria Lake which most nearly resembles unwidened river in its average rate of flow (i. e.. Lower Peoria Lake) they usually reached both their highest average numbers and their maxima in the extra-channel areas. This was noted and commented upon in 1922 and 1923, and is believed to express the preference of these worms for the ordinarily more stagnant conditions prevailing in those areas and for the conse(|uently more fre- quently renewed supply of rich bottom sediments, following freshets, in such situations. In both Upper and Middle Peoria Lakes, however, quite the reverse of these conditions is foimd to hold: both greatest average abundance and maxima occurring in recent years almost without exception in the main stream-channel rather than in the wide-waters. Both of these lakes have an average low-water rate of flow both in the steam-boat chan- nel and in the wide-waters considerably under that of the Lower Lake, and it is cjuite a])parent that sufficient sedimentation takes place within the channel itself to supply the worms with nearly if not quite optimum soil conditions. .As an additional influence on abundance of the worms in the direction noted, the fact may be mentioned that the princi|ial part of the feeding by the large commercial fishes ordinarily takes place in the areas outside the channel ; and in these two sections of Peoria Lake the close mingling of great numbers of Tiibificidac and Sphacriidac could easily result in the consumption of tmusually large quantities of the worms by bottom fishes while in quest of the bivalves. The large Sphacriidae, as represented by the single extremely abun- dant species of Peoria Lake, MuscuUnin tnmsvcrsnin, have not been ob- served to follow the rule of the small worms, but rather, with only two or three unimixirtant exceptions, reached their largest average numbers per square }ard in the period 19211 to 1925 in the deeper-channel areas, both in the three sections of Peoria Lake and in the more important 454 Illinois Natiral History Survey Billetix river reaches above and below it. This recent distribution of the Spliaeriidac as between channel and extra-channel areas quite reverses the rule that prevailed in 1!)13-1!)15, when, in the sections of river most richly supplied with those small mollusks, much the largest numbers were taken in the extra-channel areas. The rule of distribution at that time seems to have reflected, indirectly, the fact of preference for, and preemption of the channel territory by, the large Viviparidac and PJcuroccridac which have been for the most part a missing element in the small bottom fauna since li)20. Though the figures of abundance for total chironomid larvae over the period 1920 to 1925 are mixed and in many cases quite indefinite as to trend, there appears a clear tendency both in all the unwidened river reaches and in Lower Peoria Lake to reach greatest numbers in the areas outside the channel—in this respect, following the rule observed of the Tubificidae. In the Upper and Mid- dle Lakes, on the contrary, as in the instance of the worms, the highest average figures and the maxima both fell without exception in the deeper channel areas, for reasons no doubt not substantially different from those just cited as probably affecting the Tubificidae. Changes in the Mussel Fauna of Peoria Lake, 1912 to 1925 Danglade*, in the course of his examination of the Illinois River for the United States Bureau of Fisheries in 1911-1912, found a total of forty- one kinds of mussels in Peoria Lake (with Chillicothe, slightly above the upper end, included) but did not publish separate lists for the three subdivisions. Nineteen of these were commercial species regularly salable, of which at least ten were then easy to obtain in paying numbers. The mussels died out rapidly in all three sections of the lake during and after 191T until commercial clamming entirely ceased because of failure to ob- tain shells. In the summer of 1920 a single clammer operated a bar for a few days in the channel of the Lower Lake opposite the center of Peoria, but took nothing but dead shells except for an occasional live specimen of Amblcma rariplicata or still less frequently Quadrula pnstulosa. In 1920-1922 the Natural History Survey took single examples of three species (Amblcma rariplicata, Aiwdonta iiiibccillis, and Quadrula pnstu- losa) with the Petersen bottom sampler incidentally to the collection of the small bottom fauna of the Lower Lake. Commercial clamming on a scale much reduced from that of the pre-1920 period began again in 1924 and has been continued sporadically by a few clammers since. The only commercial species obtained since then in salable quantity, however, has been the common three-ridge, Amblcma rariplicata. Following out the suggestion from the commercial clammers, the Survey in the summers of 1924 and 1925 operated with a standard clammer's dip-net over fairly extensive areas between Peoria Narrows and the vicinity of Spring Bay, * Danglade, Ernest, The Mussel Re.sources of the Illinois River. Report U. S. Bureau of Fisheries for 1913, Appendix VI. pp. 1-48. BoTToir Fauna, Ii.i.inois River, 1913-1925 455 near the foot of the Upper Lake, and took, in all, during the two seasons, 16 species. Of the sixteen kinds taken, only one, the common three-ridge, as in the recent commercial clanmiing in the Lower Lake, was found in more than small and scattering numbers. The eight species taken by us in the Upper Lake in l!i'<;4 and 1925 all came from two stations at its lower end, in both cases in unusually favorable situations : either opposite the foot of Partridge Island, on the east side, and only shortly Ijelow the outlet of Partridge Creek ; or in the unusually strong current in Spring Bay Narrows. Springs under the bed of the river are known to exist in the lower end of this section of Peoria Lake, in the vicinity of Spring Ray, their occurrence there giving the name to the old village. At various stations in the ^Middle Lake, between Mossville and Tow- head Island, eleven kinds were taken in 19"21-U)ti.j. The most of the remnant beds in the Middle Lake were located on the west (or bluflf) side, but some successful drags were made as much as 500 to 700 feet from shore on either side of the channel. Springs are very frequent along the west bluflf and quite possibly exist in this section of the lake under the river and lake bed in some places. L^nless this is true, or unless these species are able to bury themselves in the nuid during the hot season for considerable periods (a supposition which is thought doubtful), it is hard to understand how even these unusually tolerant species can have survived the destruction by pollution that occurred generally in this lake between 1916 and 1920. For the recent findings, either in the Upper or Middle Lake, are not to be looked upon as any new development, the specimens having probably remained alive in substantially their recent locations through the worst of the wave of pollution that destroyed the more sensi- tive Mollitsca about ten years ago. TAlil.K XX Unionidae of Peokia Lake, 1924-1925 Upper Peoria Lake Foot of Partridge Island, Spring Bay Narrows Fusconaia nnilata QundniUi quadnihi Ainblcma rariplicata Lasmiyoria covti'Jo'ita Anodonta corpulcnta Anodonta imbecillis Lampsilis fallaciosa Lampsilis siliquoidea Middle Peoria Lake Mossville to Towhead Island, miscellaneous stations Fusconaia undata Quadrula quadnila Amblema rariplirata Anodonta corpulrnta Anodonta subo>'bicuhita Anodonta imbrcillis ObUquaria reflexa Leptodea fragilis Cai-unciiliiia parva Lampsilis fallaciosa Lampsilis siliquoidea Lower Peoria Lake Peoria Xarrows, south of bridge Fusconaia undata Quadrula pustulosa Quadrula quadrula Amblrina rariplicata EUiptio dilatatus Anodonta corpiilenta Anodonta imbecillis ObUquaria reflexa Leptodea lacvissima Leptodea frariilis Proptera alata Lampsilis fallaciosa Lampsilis siliquoidea 456 Illinois Nati'hal History Suhvey Billktin The thirteen kinds of mussels taken in 192-4-l!)25 at the head of the Lower Lake, just below the Peoria Narrows wagon bridge, require no exj)lanation, as the current is unusually swift there, and the dissolved oxy- gen under the worst conditions in the warm season usually ranges between 4 and 5 parts per million and, when the green plankton is the most abun- dant and active, sometimes exceeds 8 parts per million. Comparison with our own 1912 records of occurrence in the Illinois River above Chillicothe, when about 3 parts per million, instead of zero, as recently, was the usual lower limit of the dissolved oxygen at Chilli- cothe, shows that it is largely the same list of species now showing un- usual tolerance in Upper and Middle Peoria Lake, that ranged farthest up stream in the badly fouled river above the Upper Lake (and Chillicothe) in 1912. We find, in fact, that all of the eight species found in U]jper Peoria Lake in 1924-1925 ranged at least as far north as Hennepin (27 miles above the head of LTpper Peoria Lake) in 1912; that two of them (Aiiiblcuia rariplicafa and Lasmigona coinplanata) were taken at Starved Rock (50 miles above Chillicothe) in 1912 ; and that two others (Anodonfa corpidenta and Qitadnila quadrula) ranged then as far north as Spring Valley (38 miles above the head of the Upper Lake ). It is also noteworthy that Amblcma rariplicala, one of the two species found as far north as Starved Rock in 1912, was the only one of the entire sixteen taken in Peoria Lake in 1924-1925 that occurred in more than very scanty numbers. The 1924-1925 list of mussels from the Middle Lake had among its eleven species one that occurred as far north as Starved Rock in 1912 (Amblema rariplicafa) ; one that was taken above Peru that year ; and two that then occurred as far north as Spring Valley. Of the remaining seven, four occurred as far north as Hennepin in 1912 ; one between Chilli- cothe and Henry ; and two not at any of the 1912 collecting points. Though the conditions of current and dissolved-oxygen supply are unusually good at Peoria Narrows, it seems also that to a great extent only the hardier mussels have been able to hold out there through and since the 1917-1920 period of destructive pollution. So we find that the 1924-1925 list of thirteen kinds from there includes three species with a northward range between Peru and Starved Rock in 1912; three with northward range to Spring Valley in 1913 ; five others with occurrences as far north as Hennepin then; and the other two with older records from between Chillicothe and Henry. Combining our 1912 lists from the river between Chillicothe and Starved Rock with the 1924-1925 lists for the three sections of Peoria Lake, we now have a total of 28 species that may be regarded as sho\y- ing considerably more tolerance than the various species of middle Illi- nois River Uiiionidac not included in it. A useful subdivision of these 38 kinds on the basis of relative sensitiveness is feasible if we regard Spring Valley in 1912 as marking about the lower limit of pollutional conditions, much as those portions of the Upper Peoria Lake bed not especially protected by spring water or unusual current have done Bottom Faixa. Illinois Rivkr. 1913-1925 457 in more recent years. Subdividing the list in this way as best possible, and taking account of relative abundance, and the possibility of special protection afforded by spring water or current, we find that eleven of them may be classed as conditionally pollutional, with a single one of that group fAinblcina rariplicata) much less sensitive than the other ten; while about 17 others may be classed as possibly early or late sub- pollutional. or in other words, as more or less tolerant. In addition to the list of 28 less sensitive species immediately following, we also present here the complete list of the 41 species taken by Danglade be- tween Chillicothe and the foot of Peoria Lake under the decidedly cleaner-water conditions of 1912. Table XXI List of Least Sensithe Uxioxidae, Illinois River, With Farthest Northward Stations of OccLTtREXCE, 1912 and 1924-1925 Species Farthest north 1912 Farthest north 1924-1925 1. AmMema rariplicata Starved Rock Upper Peoria Lake, lower end 2. Lasmigona compJanata Starved Rock Upper Peoria Lake. lower end 3. Lrptodea fragilis above Peru Middle Peoria Lake 4. Proptera alata above Peru Peoria Narrows 5. Artinonais carinafa above Peru 6. Quadnila pustulosa Spring Valley Peoria Narrows 7. Quadnila quadnila Spring Valley Upper Peoria Lake S. Anodonta corpuletita Spring Valley Upper Peoria Lake. lower end 9. Megalonaias gigantea Spring Valley 10. Tritogonia tuieroulata Spring Valley 11. Anodonta grandis. var. gigantea Spring Valley 12. Fusconaia iindata Hennepin Upper Peoria Lake, lower end 13. A)wdonta imbecilUs Hennepin Upper Peoria Lake, lower end 14. Leptodea laevissima Hennepin Peoria Narrows 15. Lampsilis faJlaciosa Hennepin Upper Peoria Lake, lower end 16. Lampsilis siliguoidea Hennepin Upper Peoria Lake. lower end 17. Stropliitus edentulus Hennepin 18. Oarunculiua parva Hennepin Middle Peoria Lake 19. Lampsilis ventricosa Hennepin 20. Lampsilis occidens Hennepin 21. Anodontoides ferrusacianus.. .Hennepin 22. EUiptio dilatatus Henry to Chillicothe. . -Peoria Narrows 23. Obliguaria rcflejra Henry to Chillicothe. . .Middle Peoria Lake 24. Fusconaia ehena Henry to Chillicothe 25. Truncilla tnincata Henry to Chillicothe 26. Plnyiola lineolata Henry to Chillicothe 27. Lainpsilis anodontoides Henry to Chillicothe 28. Anodonta suborbiculata Middle Peoria Lake 458 Illinois Natural History Survey Bulletin Table XXII List of Mussels Reported hy Danglade* from Peoria Lake, Inclusive of Chillicotiie, in 1911-1912; NOiM^;NCLATURE Revised i!y Mr. P. C. Baker; Order of Danglade's List Unchanged 1. BoTTo.M F.ux\. Illinois Rivku, 1913-1925 459 tional determinations in several groups, particularly leeches, since the publication of earlier papers in this series. The complete tabulations used in making up the summaries are in the tiles nf the Natural His- tory Survey, but are tfuj voluminous and com|)k-x for present publication in detail. In the valuation tables the weights given in the case of Mollusca are those remaining after deduction of weight of shells, and in the case of all groups, after correction for an ascertained average body shrinkage in alcohol, in instances of the use of that preservative. In various tables zero is used to indicate the fact of absence in reaches where collections were made : while a blank space indicates that no collections were taken. In the valuation and abundance tables contractions of mi.xed group titles are made use of as follows at heads of the vertical columns: Sphacriidac, for same plus unimportant numbers of young or dwarf Untonidac. Gastropoda, for same plus unim])ortant numbers of Pidmonata. "Others", meaning other groups than Tubificidae, Chironoiiiidac, Sphacriidac. Leeches, and Gastropoda; and including (ex- cept in 1920, when leeches were included under this head), as the most important: burrowing Ephciucvidac ; Hydrops\- chidae ; Odoiiafa; Tiirbcllaria; and Ampliipoda. Bryo::oa. and other incrusting or attached forms, although given places in the species lists, are excluded from the quantitative data in all cases. 460 iLLixois Natiral History Survey Bulletin so \% <% o s CO < £ K So i Bottom Fauxa, Illinois River, 1913-1925 461 oi Ci Oi -^ <:o in • rH o o o m . ,-1 t-i T-t Tf lc lo o m m -^ S a, CO DO CO CO M it>«rfLO &; mmc^wco r- M in ci ^ o ^ Tji -^ m "<* CO C5 O^ Cl Oi Ci OS o M CO Tf* in o < c CO 466 Illinois Natural History Survey Bulletin ^ < O a =c BoTTOJi Fauna, Illinois River, 1913-1925 467 1-9 < O > X a 468 Illinois Natural History Survey Bulletin Bottom Fauna, Illinois River, 1913-1925 469 Summary The present paper is based on a study of 1,308 biological collections made by the Natural History Survey between 1913 and 1925, inclusive, over a 225-mile stretch of the Illinois River and in its connected bottom- land lakes. For convenience in discussion, this stretch of water is di- vided into eight reaches, as follows : Beginning at LaSalle, which is 101.5 miles below Lake Michigan, the first reach extends downstream to Chillicothe, a distance of 45 miles ; the second, from Chillicothe to the foot of Peoria Lake, 20 miles ; the third, to Copperas Creek Dam, 24 miles ; the fourth, to Liverpool, 9 miles ; the fifth, to Havana. 8 miles ; the sixth, to Beardstown, 31 miles; the seventh, to Lagrange Dam. 11.5 miles; and the eighth a distance of TT.5 miles, to Grafton at the mouth of the river. The upper part of the river is powerfully affected l)y pollution from the Sanitary District of Chicago, the eft'ect of which diminishes slowly downstream and varies considerably with the stage of water and time of year. This pollution has increased, of course, with the population of the contributing cities and with the growth and activity of certain indus- tries, the most important of which are those of the Chicago stockyards. During the six years from 1914 to 1920, tangible wastes of Chicago, in- cluding those of its stockyards, increased by about 11 per cent, and to these was added the sewage from minor but increasing sources at various points down the river, especially at Peoria and Pekin. A notable upward rush of pollutional ratios and effects, due to war-time activities of the stockyards, culminated in 1918 in a sewage contribution from this source equivalent to that of a population of 1.390,000 people, but fell oiif during the first two years of peace by about 25 per cent. An elTort is made to distinguish septic, pollutional. sub-pollutional, and clean-water conditions by the plant and animal species characteristic ' of each, and from these to select an index series of species whose abun- dance marks the grade of pollution in which each species predominates ; but this classification of degrees of pollution is difticult l)ecause the several divisions fade into each other gradually with no well-marked boundaries, and different grades intermingle in the same locality, those of the main current of the stream differing from those dominating in comparatively sluggish, shallow, and weedy marginal waters ; and these dift'ering again from those of still more sluggish, broad expanses of shallow water. Even in the channel of the river itself the organisms suspended in the flowing stream dift'er. often widely, both in relative abundance and in sensitivity, from those of the bottom sediments, and the conclusion is reached that a dependable classification can be arrived at only by the supplementing of chemical data with a comprehensive study of the dominant fauna and flora in each situation. Increase of pollution between 1913 and 1920 is shown by the advance downstream of stages of pollution. Clean-water conditions, which in 1913 extended upstream within 53 miles of LaSalle, had receded by 1920 to a point 53 miles still farther down, an average recession of about 8 miles a year. 470 Illinois Natural History Survey Bulletin Deterioration of the condition of the river bottom during this inter- val is shown also by a reduction in the number of species represented in the bottom fauna, amounting to an average of 64 per cent in the Peoria Lakes, 83 per cent in the river between these lakes and Havana, and 48 per cent between Havana and Beardstown. Another measure of increased pollution is found in the number of characteristically clean-water species that disappeared during the same interval. The Peoria Lakes, for example, were entirely cleared of these species by 1920. In the 49 miles next below they had been reduced from 49 species to three, and in the next 31 miles from IT species to six. Cor- responding data are given for an increase in the number of pollutional and sub-pollutional species dvn-ing the same interval and over the same sec- tions of the river, but there was some slight recovery of clean-water species after the end of the world war. The dissolved oxygen data con- stitute a similar record of rapid decline from 1912 to 1920 and of a par- tial recovery in the Peoria Lakes by 1925. The effect of increasing pollution upon the more important groups of the bottom fauna is reflected in a general and often very large decrease in average numbers per square yard in all of these groups except the pollutional sludge worms (Tubificidae) and midge larvae (Chironomus), which multiplied enormously in this period. In a 10-mile section of the river just above Havana, for example, where there were 1.709 Sphaeriidae (small bivalve mollusks) per square yard in 1915, there were only 46 per square yard in 1920 and the numbers of Gastropoda (snails) declined from 496 to 20; but where there were 16 sludge worms and 10 midge larvae per square yard in 1915 there were 2,463 sludge worms and 733 midge larvae in 1920, and in the following four or five years the niunbers of sludge worms had still further multiplied to 39,183 per square vard. From 1920 to 1925, however, the Sphaeriidae, which had been diminish- ing so rapidly under the heavier pollution of the war period, recovered quickly as pollution diminished, rising in the 10-mile section just men- tioned from 46 per square yard in 1920 to 12,785 in 1924. This uprising of Sphaeriidae appears to have been checked and reversed by an epidemic outbreak of predaceous leeches preying on them, the leeches themselves multiplying from 441 to 10,275 per square yard, while the Sphaeriidae fell from 12,785 to 6,465. Valued by total weights instead of numbers of orgaiiisius, the small bottom fauna (mostly gastropod mollusks) ranged in 1915 from 170 to 360 pounds per acre between Chillicothe and the Lagrange Dam except in a 17-mile section above Havana where it rose to 1,000 pounds per acre in the upper 8 miles and 2,700 pounds in the lower 9 miles; but by 1920 these snails were virtually exterminated in the Peoria Lakes, where, on the other hand, the sludge worms and midge larvae of a polluted water rose to 86 per cent of the total weight. Between Liverpool and Havana gastropod mollusks were reduced to 19 per cent of the average haul, and midge larvae were increased to 53 per cent. Bottom Fai na. Ili.ixois RnEu. 1913-1925 471 Following upon prolonged and destructive floods in the summer of 1924. which presumably carried down and distributed great quantities of up-the-river sludge and its inhabitants, there was an enormous increase of the total product made up mostly of sludge worms, midge larvae, and MuscuUiim transvcrsKiii (a little bivalve moUusk tmusually tolerant of pollutional conditions ) . In the three Peoria Lakes and in the Liverpool- to-Havana section of the river, the total weight rose in lii24 to 25 or 26 times that of 1920, composed in the lakes almost wholly of the little Musculiimi, which made 80 per cent to 91 per cent of the product of an average haul. Study of the competitions and depredations of the more important groups of the bottom animals shows that under clean-water conditions snails, bv their feeding methods, tend to dominate and, where they be- come abtmdant, to suppress midges, sludge worms, and the smaller Sphaeriidae, and that sucker-mouth fishes draw heavily upon small mol- lusks and larvae of midges, mayflies, and caddis flies, preferring these to sludge worms, leeches, and the larger snails. Only catfishes (and sheejxsheads ) prey extensively upon the larger snails. The destniction of the latter by pollution and the release by this means of the small Sphaeriidae affords to "coarse fish" an increased available food supply and so may be a benefit rather than a detriment to commercial fisheries. On the other hand, the frequently gassy taste and, in the case of the carp, the diseased condition of as many as 50 per cent, due to the effects of pollution upon their food, diminishes the market value of Illinois River fishes. The numbers of midge larvae and of Musculium were at first di- minished and afterwards increased by the floods of l!t24, and the numbers of sludge worms were at first increased and afterwards diminished. These contrasting variations and those of Musculium, as a consequence of the floods, are described and discussed in detail with suggested explanations ; and a study is reported of the comparative abundance of these same groups in the river channel and in the shallow waters adjoining. The effect of pollutional conditions on the river mussels is shown by a com- parison of a list of 41 species found in Peoria Lake in 1911 and 1912 with the 15 species remaining in 1924 and 1925. eight of them in the U])per lake, eleven in the middle, and thirteen in the lower. The lag of the small bottom animals behind the dissolved oxygen (and also the plankton and bacteria of the epilimnion ) appears to be due jjrincipally to two causes—the naturally slow rate of re-spread up- stream of cleaner-water forms, as against the easier downstream move- ment of the pollutional and unusuallv tolerant bottom species : and the delivery, with every flood, into territory that would otherwise remain reasonably clean, of fresh loads of incompleteh- oxidized organic sedi- ment. From the preceding statement, and others in the body of this paper, the inference is drawn that the small bottom animals, except where the pollution is very heavy, on the whole furnish a better index of the funda- 472 Illinois Natural History Survey Bulletin mental or permanent sanitary condition than the frequently rapidly chang- ing dissolved oxygen or plankton. Bearing in mind the popular distinction between "coarse" fishes and "fine" fishes, which may usefully be extended for the moment to the small bottom animals, it is to be emphasized that the increases in pounds-per- acre averages between Chillicothe and Beardstown since 1920 represent almost wholly enlargement in quantity at the expense of quality, and have occurred for the most part without corresponding permanent improvement in sanitary condition. Such food is available only to the fishes which are able to live under the conditions prevailing where it is produced ; and large portions of those areas are still subject in the warm season to spells of oxygen depletion that are likely to exclude from these rich feeding grounds all except the most tolerant of the bottom feeding fishes. Bottom Fauna, Illinois River, 1913-1925 473 APPENDIX TO ARTICLE XII The following is a list of publications of the State Natural History Sur- vey and its predecessor, the State Laboratory of Natural History, dealing in whole or in part with investigations of the Illinois River. The asterisk marks articles relating to Illinois River biology as such, to distinguish them from others of a more general nature which contain some information on this subject. Bulletin Series 1876. List of Illinois Crustacea. S. A. Forbes. Vol. 1, No. 1, pt. 1. (25 pp., 1 pl.) 1876. A partial catalogue of the fishes of Illinois. E. W. Nelson. Vol. I, No. 1, pt. 4. (30 pp.) 1877. A catalogue of the fishes of Illinois. David Star Jori).\n. Vol. I, No. 2, pt. 4. (34 pp.) 1S77. The food of Illinois fishes. S. A. Forbe.s. Vol. I, No. 2, pt. 5. (19 pp.) ISSO. The food of fishes. S. A. Forres. Vol. 1, No. 3, pt. 2. (48 pp.) 1880. On the food of young fishes. S. A. Forbes, Vol. I, No. 3, pt. 3. ( 14 pp.). Second edition. HXi.l. Reiirint. JOl'J. 1883. The food of the smaller fresh-water fishes. S. A. Forbes. Vol. I, No. 6, pt. 3. (30 pp.) 1888. Studies of the food of fresh-water fishes. S. A. Forbes. Vol. II, Art. 7. (41 pp.) 18S8. On the food relations of fresh-water fishes: a summary and dis- cussion. S. A. Forbes. Vol. II, Art. 8. (63 pp.) *1S95. On the entomology of the Illinois River and adjacent waters. C. A. Hart. Vol. IV, Art. 6. (125 pp., 12 pl.) *1S96. Descriptions of new species of Rotifera and Protozoa from the Illi- nois River and adjacent waters. Adolph Hempei,. Vol. IV, Art. 10. (8 pp., 5 pl.) 1897. Contribution to a knowledge of the North American fresh-water Ostracoda included in the families Cytheridae and Cyprididae. Richard W. Sharpe. Vol. IV, Art. 15. (71 pp., 10 pl.) *1897. Plankton studies. I. Methods and apparatus in use in plankton in- vestigations at the Biological Experiment Station of the University of Illinois. C. A. Kofoid. Vol. V, Art. 1. (25 pp. 7 pl.) 1897. A contribution to a knowledge of the North American fresh-water Cyclopidae. Ernest B. Forbes. Vol. V, Art. 2. (56 pp., 13 pl.) 1897. The North American species of Diaptomus. F. W. Sciiacht. Vol. V, Art. 3. (Ill pp., 15 pl.) *1898. Plankton studies. II. On Phodorina illinoiscnsis. a new species from the plankton of the Illinois River. C. A. Kopoid. Vol. V, Art. 5. (21 pp., 2 pl.) *1S99. A list of the Protozoa and Rotifera found in the Illinois River and adjacent lakes at Havana, Illinois. Adolph Hempel. Vol. V, Art. 6. (88 pp.) *1899. A statistical study of the parasites of the Unionidae. H. M. Keij,y. Vol. V, Art. S. (20 pp.) 474 Illinois Natuual History Survey Bulletin *1899. Plankton studies. III. On Platydorina, a new genus of the fam- ily Volvocidae, from the plankton of the Illinois River. C. A. Kofoid. Vol. V, Art. 9. (22 pp., 1 pi.) 1901. The Hirudinea of Illinois. J. Percy Moore. Vol. V, Art. 12. (69 pp., 6 pi.) *1903. Plankton studies. IV. The plankton of the Illinois River, 1894- 1899, with introductory notes on the hydrography of the Illinois River and its basin. Part I. Quantitative investigations and general re- sults. C. A. KoFOiD. Vol. VI, Art. 2. (535 pp., 50 pi.) 1904. A review of the sunflshes of the current genera Apomotis, Lepomis, and Eupomotis, with particular reference to the species found in Illinois. R. E. Rkhardsox. Vol. VII, Art. 3. (9 pp.) 1906. A catalogue of the Mollusca of Illinois, F. C. Baker. Vol. VII, Art. 6. (84 pp., 1 map.) 1907. On the local distribution of certain Illinois fishes: an essay in statistical ecology. S. A. Forhes. Vol. VII, Art. 8. (31 pp., 15 maps, 9 pl.) *1908. The plankton of the Illinois River. 1894-1899, with introductory notes upon the hydrography of the Illinois River and its basin. Part II. Constituent organisms and their seasonal distribution. C. A. Kofoid. Vol. VIII, Art. 1. (360 pp., 5 pl.) *1913. Observations on the breeding of the European carp in the vicinity of Havana, Illinois. R. E. Richardson. Vol. IX, Art. 7. (19 pp., 1 map.) *1913. Observations on the breeding habits of fishes at Havana, Illinois, 1910 and 1911. R. E. Richakd.son. Vol. IX, Art. 8. (13 pp., 1 pl.) *1913. Studies on the biology of the upper Illinois River. S. A. Forbes and R. E. Richardson. Vol. IX, Art. 10. (95 pp., 21 pl.) *1915. The Chironomidae, or midges, of Illinois, with particular reference to the species occurring in the Illinois River. John R. Malloch. Vol. X, Art. 6. (269 pp., 24 pl.) 1918. Ways and means of measuring the dangers of pollution to fisheries. Victor E. Shelfohd. Vol. XIII, Art. 2. (18 pp.) *1919. Some recent changes 'in Illinois River biology. S. A. Forbes and R. E. Richardson. Vol. XIII, Art. 6. (18 pp.) *1919. Acanthocephala from the Illinois River, with descriptions of species and a synopsis of the family Neoechinorhynchidae. H. J. Van Cleave. Vol. XIII, Art. 8. (33 pp., 7 pl.) *1921. The small bottom and shore fauna of the middle and lower Illinois River and its connecting lakes, Chillicothe to Grafton: its valuation; its sources of food supply; and its relation to the fishery. R. E. Richardson. Vol. XIII, Art. 15. (161 pp.) *1921. Changes in the bottom and shore fauna of the middle Illinois River and its connecting lakes since 1913-1915 as a result of the increase, southward, of sewage pollution. R. E. Richardson. Vol. XIV, Art. 4. (43 pp.) 1923. The determination of hydrogen-ion concentration in connection with fresh-water biological studies. Victor B. Shelford. Vol. XIV, Art. 9. (17 pp.) *1925. Changes in the small bottom fauna of Peoria Lake, 1920 to 1922. R. E. Richardson. Vol. XV, Art. 5. (61 pp.) Bottom Pauxa. Illinois River. 1913-1925 475 *1925. Illinois River bottom fauna in 1923. R. E. Richardsox. Vol. XV, Art. 6. (31 pp.) *1925. Some observations on the oxygen requirements of fishes in the Illi- nois River. David H. Thompson. Vol. XV, Art. 7. (15 pp.) 192S. The biological survey of a river system—its objects, methods, and results. S. A. Forbes. Vol. XVII. Art. 7. (8 pp.) *192S. The "knothead" carp of the Illinois River. David H. Thomi-sox. Vol. XVII, Art. 8. (36 pp.) *1928. The bottom fauna of the middle Illinois River. 1913-1925; its dis- tribution, abundance, valuation, and index value in the study of stream pollution. R. E. Rhh.vrdsox. Vol. XVII. Art. 12. (86 pp.) Reports of the Director of the Illinois State Laboratory of Natural History S. A. Forbes, Director 1888. Report for 1887-1S8S, pp. 2-4: General program of aquatic zoology. 1894. Report tor 1S93-1S94. pp. 3-26: Zoological exhibit at the Columbian Exposition, establishment of the Illinois River biological station at Havana, etc. (15 pi.) *1896. Report for 1895-1896. pp. 7-31: Special report of the biological ex- periment station. (20 pi.) *1S98. Report for 1897-1898. pp. 4-7: Personnel, equipment, summer school, etc.; also pp. 8-25: Report of the superintendent of the biological station, C. A. Koroin; pp. 25-27: Report on water analysis, by Arthxh W. Palmer, professor of chemistry: and pp. 29-31: Report on the summer school of 1898. by Pkamc S-Mitii. assistant proftssor of zo- ology. (10 pi.) 1901. Report for 1S99-1900. pp. 3-11: Study of Illinois fishes and care of collections; study of the plankton, aquatic insects, and leeches; ex- pansion of the library and its exchange list; etc. 1915. Report for 1913-1914, pp. 7-10: Continuation of operations on the Illinois River and its tributary waters. Final Reports on the Natural History Survey of Illinois 1908. The fishes of Illinois. S. A. Formes and R. E. Rkhakdsox. Vol. Ill of the final reports on the natural history survey of Illinois. (cxxxvi-f357 pp.. 68 pi., 76 text fig.; 103 maps in a separate atlas ) Second c(litio)i. I'J^n.