GEOLOGY OF THE INTERMOUNTAIN WEST an open-access journal of the Utah Geological Association ISSN 2380-7601 Volume 10 2023 This is an open-access article in which the Utah Geological Association permits unrestricted use, distribution, and reproduction of text and figures that are not noted as copyrighted, provided the original author and source are credited. Email inquiries to GIW@utahgeology.org. SNOW DROUGHT AND MONSOON FLOODS: HYDROLOGICAL EXTREMES IN THE CEDAR VALLEY WATERSHED DURING WATER YEAR 2021, SOUTHWESTERN UTAH Erich R. Mueller, Garrett P. Sudweeks, Shadrach A. Ashton, and Micah C. Olson Theme Issue Engineering Geology and Geohazards of Utah Utah Geological Association Annual Field Conference October 22, 2022 Salt Lake City, Utah, USA GEOLOGY OF THE INTERMOUNTAIN WEST an open-access journal of the Utah Geological Association ISSN 2380-7601 Production Cover Design and Desktop Publishing Douglas A. 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Foster Utah Field House of Natural History State Park Museum 435.789.3799 eutretauranosuchus@ gmail.com GIW Editors UGA Field Conference Editors Jason Kaiser Southern Utah University jasonkaiser@suu.edu John South Geologist johnvsouth@gmail.com Adam McKean Utah Geological Survey adammckean@utah.gov Rick Chesnut Terracon Rick.Chesnut@terracon.com GEOLOGY OF THE INTERMOUNTAIN WEST an open-access journal of the Utah Geological Association Volume 10 2023 93 ABSTRACT Water year 2021 was a year of extremes in the Cedar Valley watershed of southern Utah, with snow drought resulting in extremely low snowmelt runoff of Coal Creek and intense monsoon rainfall resulting in several floods in different parts of the valley. Winter snow accumulation was depressed throughout southern Utah, perhaps due to La Nina conditions affecting winter storm trajectories. Coal Creek, the principal stream providing surface water to Cedar Valley, typically receives most of its annual flow from snowmelt runoff, but in 2021 had a peak snowmelt discharge 15% to 25% of that recorded in the previous two years and the third lowest snowmelt runoff on record. Following this extremely low snowmelt runoff period, more than 10 floods of Coal Creek occurred following monsoon storms in July and August that ex- ceeded the 2021 snowmelt peak. Additionally, several thunderstorms produced rainfall rates in exceedance of the 100-year event and induced flooding within Cedar City and the town of Enoch. Flood inundation modeling using HEC-RAS and high-resolution topographic data showed good agreement with field and pubic-survey data on the high-water stage during the Enoch flooding, and demonstrated that the flooding was likely exacerbated by the low topography and limited drainage potential in flooded areas. Whereas the monsoon storms improved soil moisture and helped alleviate drought conditions, they also resulted in urban flooding and did little to replenish the regional water supply. Snow Drought and Monsoon Floods—Hydrological Extremes in the Cedar Valley Watershed During Water Year 2021, Southwestern Utah Erich R. Mueller1,2, Garrett P. Sudweeks1,3, Shadrach A. Ashton1,4, and Micah C. Olson1,5 1Department of Geosciences, Southern Utah University, Cedar City, UT 84720 USA 2erichmueller@suu.edu, 3garrett@kairosgeodetics.com, 4shadashton@gmail.com 5olsonmicah86@gmail.com Citation for this article. Mueller, E.R., Sudweeks, G.P., Ashston, S.A., and Olson, M.C., 2023, Snow drought and monsoon floods—hydrological extremes in the Cedar Valley watershed during water year 2021, southwestern Utah: Geology of the Intermountain West, v. 10, p. 93–111, https://doi.org/10.31711/giw.v10.pp93-111. INTRODUCTION Streamflow and flooding in southern Utah occur through a combination of snowmelt, rainfall, and rain- on-snow (Lund, 1992; Wilkowske and others, 2006; Lund and others, 2010). For perennial or intermittent streams draining higher elevation terrain, groundwater inputs contribute the seasonal baseflow, but floods typi- cally occur from more rapid surface runoff during snow- melt or intense rain storms. From a management per- spective, streams and rivers provide a major component of the water supply as both surface runoff and a source of groundwater recharge to downstream basins, but also present a hazard as floods are common throughout the region and often impact developed areas. For larg- er watersheds with headwaters in higher elevation ter- rain, snowmelt plays an important role in the regional water supply because most streamflow is derived from direct snowmelt runoff and snowmelt-derived recharge 94 Snow Drought and Monsoon Floods—Hydrological Extremes in the Cedar Valley Watershed During Water Year 2021, Southwestern Utah Mueller, E.R., Sudweeks, G.P., Ashston, S.A., and Olson, M.C. Geology of the Intermountain West 2023 Volume 10 of groundwater and stream baseflow (Julander and Clayton, 2018). On the other hand, the largest floods are typically generated by convective thunderstorms in the late summer and early fall or via winter rain events (Wilkowske and others, 2006). For example, the Vir- gin and Santa Clara Rivers that merge near St. George, Utah, have experienced large floods due to warm Pa- cific storm systems interacting with terrain to produce sustained rain and rain-on-snow at higher elevations (Bardsley and Julander, 2005; Wilkowske and others, 2006). Flash floods from intense convective thunder- storms also cause flooding across basins of many scales, but often have their greatest impact on smaller water- sheds due to the often-isolated nature of storm develop- ment (Woolley, 1946; Berwick, 1962; Smith and others 2019); these types of “cloudburst” floods have long been recognized as a frequent cause of flooding throughout Utah (Woolley, 1946). In this study, we explore the hydrological events during water year 2021 in Cedar Valley, Utah, that both exacerbated drought conditions due to a lack of signif- icant snowmelt, and caused extensive flooding during multiple events within different areas of the valley. We use long-term hydrological records from Coal Creek, the principal stream and primary water source draining into Cedar Valley, to document the long-term water- shed hydrology and evaluate water year 2021 relative to nearly 100 years of streamflow measurements. We also use precipitation data to estimate return frequencies for three flood events that occurred due to convective thunderstorms, and hydraulic modeling to evaluate the flood impacts in an urbanized area during one of those events. Cedar City is one of the fastest growing microp- olitan areas in Utah, and a limited water supply and po- tential for flooding in developing areas are prominent concerns. Within Cedar City, numerous flood-control projects have been implemented, such as levees and di- version structures, to alleviate flooding during periods of high streamflow. Additionally, several projects have been implemented so that any excess streamflow can be diverted into basins for aquifer recharge. Water year 2021 provides a perspective on which to view poten- tial challenges in water management for Cedar City and Cedar Valley in terms of both drought and floods. BACKGROUND Hydrology of the Cedar Valley Watershed The Cedar Valley watershed is a closed basin in east- ern Iron County, Utah. The watershed extends from the Harmony Hills and Markagunt Plateau in the south, to the Black Mountains in the north (Brooks and Mason, 2005). The valley is a graben bounded by normal faults associated with Basin and Range extensional faulting, and all streamflow drains internally toward Quichapa Lake in the southern part of the basin and Rush Lake in the northern part of the basin (Brooks and Mason, 2005). The primary surface water source to the val- ley and largest of the contributing watersheds is Coal Creek, which debouches from the Markagunt Plateau at Cedar City (figure 1). The headwaters of Coal Creek occur at the high elevation rim (greater than 10,000 feet) of the plateau in Cedar Breaks National Monu- ment. As a result, significant winter snowfall feeds the perennial flow of Coal Creek and produces the bulk of the streamflow (Julander and Clayton, 2018). Aside from Shirts Creek, which is considerably smaller than Coal Creek, the remaining watersheds emerging from the mountainous areas along the edges of the valley are ephemeral (Brooks and Mason, 2005). Whereas some of these watersheds may experience a minor snowmelt pulse some years, they are most likely to generate signif- icant streamflow and flooding during convective sum- mer thunderstorms. Coal Creek has a long history of flooding; the first recorded flood occurred on Septem- ber 3, 1853, and caused extensive damage in Cedar City (Lund, 1992). Most of the development in the valley is focused near Cedar City and the nearby town of Enoch, and the focus of this study is on the streamflow hydrol- ogy of Coal Creek and the precipitation climatology of flooding during rain events affecting those towns. For the Coal Creek watershed, the largest volume of water is supplied during the snowmelt period that typically lasts from spring through early summer. Alter- natively, the largest magnitude floods occur during in- tense convective rain events in the late summer and ear- ly fall as part of the North American Monsoon. Floods from these storms can cause flooding of Coal Creek or any number of other watersheds and ephemeral streams in the area. For Coal Creek, this imparts a somewhat 95 Snow Drought and Monsoon Floods—Hydrological Extremes in the Cedar Valley Watershed During Water Year 2021, Southwestern Utah Mueller, E.R., Sudweeks, G.P., Ashston, S.A., and Olson, M.C. Geology of the Intermountain West 2023 Volume 10 unique hydrology with respect to water resources and hazard mitigation: wherein the water supply is depen- dent on snow during the winter and the resulting snow- melt flood pulse, the primary flood hazards result from short-duration and typically localized intense precipi- tation during the monsoon season. During the period of record, significant floods of Coal Creek have not occurred during the winter, unlike in the Virgin River system where rain-on-snow events are more common and the mean basin elevation is significantly lower. Coal Creek has streamflow records dating back to 1916, and provides important context to evaluate the unique hy- drology of 2021. Effects of El Nino Southern Oscillation on Winter Precipitation Previous studies have shown a correlation between the phases of the El Nino southern oscillation (ENSO) and winter precipitation in the western United States (Redmond and Koch, 1991; Cayan and others, 1999; Clark and others, 2001; Jin and others, 2006; Tamaddun and others, 2017). In particular, areas in the southwest- ern United States tend to experience increased winter precipitation during El Nino years and decreased winter precipitation during La Nina years (Brown and Comrie, 2004). The opposite signal is apparent in the northwest- ern part of the United States. Importantly, ENSO phases Figure 1. Map of study area showing the locations of the precipitation and streamflow measurement stations used in this study; MesoWest and SNOTEL site names are labeled. The Coal Creek watershed is outlined upstream from the U.S. Geolog- ical Survey stream gage just upstream from Cedar City. 96 Snow Drought and Monsoon Floods—Hydrological Extremes in the Cedar Valley Watershed During Water Year 2021, Southwestern Utah Mueller, E.R., Sudweeks, G.P., Ashston, S.A., and Olson, M.C. Geology of the Intermountain West 2023 Volume 10 in June through November can be a predictor of the subsequent winter snowpack and spring runoff (Red- mond and Koch, 1991; Oubeidillah and others, 2011). Weak La Nina conditions preceded the winter snow ac- cumulation season in water year 2021, and we explore the influence of El Nino/La Nina cycles on snow accu- mulation and streamflow for the Coal Creek watershed. North American Monsoon and Summer Precipitation The North American Monsoon (hereafter, mon- soon) is a seasonal change in the predominant wind- flow direction and atmospheric circulation associated with summer heating of the interior of the North Amer- ican continent (Adams and Comrie, 1997). Continental heating causes a low-pressure cell to develop over the high elevation interior of the western United States and induces flow of warm and moist air from the Gulf of California and the Gulf of Mexico (Hereford and Webb, 1992). As a result, daily convective storms are common in much of northern Mexico and the southwestern Unit- ed States due to diurnal heating, and are responsible for numerous flash floods. As Utah is at the periphery of its strongest influence, the year-to-year variability in monsoon intensity can be large. For example, the 2020 monsoon season saw relatively little storm activity and flash flooding in most of southern Utah, whereas the 2021 monsoon season caused widespread flash flooding in much of the southwest (National Oceanic and Atmo- spheric Administration, 2021). Meteorologically, the monsoon period begins June 15th, but monsoon thun- derstorm activity is greatest from July through Septem- ber, and three floods caused by intense monsoon rains in water year 2021 are evaluated in this study. METHODS Streamflow and Snow Hydrology of the Coal Creek Watershed Coal Creek is the primary surface water source for Cedar Valley (figure 1), and U.S. Geological Survey (USGS) stream gage 10242000 is located just upstream from the mouth of the canyon east of Cedar City. Up- stream from the stream gage, the watershed is effective- ly unregulated, with no major water diversions or res- ervoirs. As such, the gage provides a useful data source to assess the trends in the long-term natural hydrology of the watershed, with annual flood magnitudes report- ed back to 1916. For the streamflow, precipitation, and snowpack data described herein, we use English units consistent with those reported by government agen- cies and commonly used for resource management and hazard assessment. Furthermore, in all cases, we use the water year, which extends from October 1st through September 30th, as the hydrologically relevant time scale over which to assess annual patterns. We consid- er the March 1st through June 30th period as generally associated with snowmelt, and the July 1st through Sep- tember 30th period as reflecting monsoon floods over- printed on the baseflow. Streamflow Volumes We use the USGS daily data from the Coal Creek gage to evaluate the long-term trends of streamflow vol- ume. We report the total annual runoff volume (per wa- ter year), the March 1st through June 30th (snowmelt) runoff volume, and the July 1st through September 30th (monsoon plus baseflow) runoff volume. Flood Frequency Analysis A flood frequency analysis using the standard USGS Log-Pearson III approach, with a gage specific skew, was used to estimate return periods for floods of different magnitude (e.g., Kenney and others, 2007). Because the stream is affected by both snowmelt floods and mon- soon-induced flash floods, we used multiple approaches to evaluate flood return periods. First, we use the maxi- mum annual flood for each year, regardless of whether it was generated by snowmelt or summer rainfall (Kenney and others, 2007). We then evaluated the gage record to find the maximum snowmelt and maximum monsoon rainfall flood for each year to examine the annual flood frequencies for the different flood-generating mecha- nisms (e.g., Yu and others, 2022). For the period since 1988, instantaneous values (ev- ery 15 minutes) of discharge are available, and we sim- ply select the highest flows in the snowmelt and mon- soon time periods (as defined above) for the different 97 Snow Drought and Monsoon Floods—Hydrological Extremes in the Cedar Valley Watershed During Water Year 2021, Southwestern Utah Mueller, E.R., Sudweeks, G.P., Ashston, S.A., and Olson, M.C. Geology of the Intermountain West 2023 Volume 10 flood types (snow versus rain). For the period prior to 1988, we use the maximum daily discharge during the spring period to estimate the peak snowmelt flood to the beginning of the period of record. For monsoon rainfall floods, the daily discharge will underestimate flood peaks because of the rapid rise and fall of the hy- drograph. When the monsoon flood was the peak for the year, we use the (instantaneous) peak flow value reported by the USGS. For other years, we extend the gage record of rain-induced floods by using a relation between daily and instantaneous values for the 1988– 2021 period. This provides a conservative estimate of these floods, and allows us to establish a longer record of rain-induced flood frequencies using the standard USGS approach that includes years with smaller mon- soon floods. Snowpack Because snowmelt is the dominant driver of the streamflow (water supply) of Coal Creek, we evaluate the historical variability in snowpack as a complement to our streamflow analysis. We use data from snow te- lemetry (SNOTEL) sites operated by the U.S. Depart- ment of Agriculture (USDA) Natural Resources and Conservation Service (NRCS) that are within or directly adjacent to the headwaters of the Coal Creek watershed (figure 1). We chose sites with long-term daily records dating back to water year 1981: the Webster Flat, Kolob, Midway, and Castle Valley SNOTEL sites (figure 1). For each site, we found the maximum snow water equiva- lent (SWE) reported for each site for each year, with the expectation that the maximum SWE value represents the water available for runoff in the subsequent spring and early summer. Soil Moisture Soil moisture data at SNOTEL sites were assessed for the period 2019–2021. Soil moisture is known to have an effect on streamflow generation in snowmelt-dom- inated watersheds of Utah (Clayton, 2016; Harpold and others, 2017). In this case, we used data from the four long-term SNOTEL sites above, and two addition- al nearby sites installed in the 2010s: Brian Head and Southern Utah University (SUU) Ranch. Soil moisture is reported as volumetric water content at 2-, 8-, and 20-inch depths, and we focus on the 8- and 20-inch data as more representative of the long-term and depth-in- tegrated soil moisture likely to affect streamflow. We report values as the percent of the median from the 2019–2021 period of analysis. Southern Oscillation Index (SOI) We use the average of the June through November standardized Southern Oscillation Index (SOI) to assess the impact of ENSO cycles on the snow and streamflow hydrology of Coal Creek (Redmond and Koch, 1991). The SOI is a measure of the difference in sea-level pres- sure of the equatorial Pacific, and is thus a measure of the relative strength of ENSO. El Nino years occur when the SOI is less than -0.5; whereas, La Nina years occur when the SOI is greater than 0.5. Other years are con- sidered Neutral. Since 1951, about half the years have been Neutral, and the other half were either El Nino or La Nina years. We group maximum SWE values and annual March–June runoff into El Nino, Neutral, or La Nina categories based on the previous year SOI values, and use Student’s T-Tests to test for statistical differenc- es based on the ENSO conditions. Precipitation Hydrology of Monsoon Flood Events in 2021 Precipitation data to evaluate the monsoon flooding in Cedar Valley was derived from a variety of sources. The primary precipitation events evaluated were the July 26th flooding in Cedar City, the August 1st flood- ing in Enoch, and the August 18th and 19th flooding of Coal Creek (figure 2). National Oceanic and At- mospheric Administration (NOAA) radar reflectivity data available from the National Centers for Environ- mental Information were used to document the extent and duration of the precipitation events. Additionally, a NOAA National Weather Service station is located near the Cedar City Regional Airport. This weather sta- tion recorded significant precipitation on July 26th, but otherwise registered minimal precipitation during the other events. Thus, we gathered data from any publicly available precipitation records for those events, as de- scribed below. 98 Snow Drought and Monsoon Floods—Hydrological Extremes in the Cedar Valley Watershed During Water Year 2021, Southwestern Utah Mueller, E.R., Sudweeks, G.P., Ashston, S.A., and Olson, M.C. Geology of the Intermountain West 2023 Volume 10 Figure 2. (A) Flooding of residential areas and (B) along Cross Hollow Drive in Cedar City on July 26, 2021 (courtesy of St. George News/STGnews.com). (C) Residents survey flooding along Midvalley Road in Enoch on August 1, 2021 (courtesy of St. George News/STGnews.com). (D) Backyard flooding with a damaged fence in the background in Enoch on August 1, 2021 (courtesy of an Enoch resident). (E) Street flooding in Enoch on August 1, 2021 (courtesy of Lisa Baker). (F) Flooding along Coal Creek east of Cedar City near the USGS stream gage and (G) in Cedar City near Main Street on August 18, 2021 (courtesy of St. George News/STGnews.com). All photographs used with permission. 99 Snow Drought and Monsoon Floods—Hydrological Extremes in the Cedar Valley Watershed During Water Year 2021, Southwestern Utah Mueller, E.R., Sudweeks, G.P., Ashston, S.A., and Olson, M.C. Geology of the Intermountain West 2023 Volume 10 MesoWest Data MesoWest is a University of Utah program that col- lects weather data from a variety of public sources and provides access to the historical data records. Four Me- soWest sites were used in this study (figure 1), with two of the sites located near the headwaters of Coal Creek and two sites located near its mouth. These sites were used to evaluate precipitation during the August 18–19 flooding of Coal Creek, with data typically reported at 15-minute intervals. None of the MesoWest sites in the areas of Cedar City or Enoch reported useable data during the July 26th and August 1st floods. SNOTEL Data Precipitation data from four SNOTEL sites (SUU Ranch, Brian Head, Webster Flat, and Midway Flat) were combined with the MesoWest data to evaluate rainfall rates during the August 18–19 flooding. SNO- TEL data are reported at 1-hour intervals. Precipitation rates at SNOTEL sites compared very favorably to pre- cipitation rates measured at nearby MesoWest sites. Weather Underground Data Because of the dearth of precipitation data available from government agencies or MesoWest for the July 26th and August 1st flood events, we utilized publicly available precipitation data from the Weather Under- ground website. Weather Underground has compiled a network of thousands of Personal Weather Stations (PWS) and provides real-time data access to the net- work. Further, they provide detailed information on compatible weather stations and station siting to help ensure the accuracy of the data provided. Whereas indi- vidual weather stations may not have been profession- ally installed, information on the weather station man- ufacturer and precision and accuracy specifications are available for each site. In total, nine PWS stations from the Weather Underground site were used to evaluate precipitation rates. Typically, these data were reported at 5-minute intervals. Precipitation Probability Data Measured precipitation rates for the sites above were compared to point estimates of depth-dura- tion-frequency precipitation probability estimates from NOAA (Bonnin and others, 2011). Probability tables were downloaded from the NOAA Atlas 14 point pre- cipitation frequency estimates website based on the latest revision of the Atlas in 2011 (Bonnin and others, 2011). The estimates are based on a partial duration se- ries, meaning that all large events above a threshold are included in the analysis, regardless of the year in which they occurred (as opposed to an annual maximum se- ries). The reported values are then based on the highest n number of events for a period of record of n years (Bonnin and others, 2011). Hydraulic Modeling with HEC-RAS We used the U.S. Army Corps of Engineers Hydrau- lic Engineering Center–River Analysis System (HEC– RAS) hydraulic model to evaluate the August 1st flood- ing in Enoch. HEC-RAS solves the two-dimensional (2D), vertically averaged, shallow-water equations to model unsteady flow across channels and floodplains as defined by a digital terrain model that quantifies the topography in the model domain. The Utah Geospa- tial Resource Center (UGRC) and the Utah Division of Emergency Management acquired lidar over Enoch, Utah from May 2nd through May 25th, 2020. Half-me- ter bare earth digital elevation models (DEMs) and first return digital surface models (DSMs) were produced and published by UGRC. Aerial photos and lidar aerial surveys with a Real-time kinematic positioning (RTK) enabled airframe and base station, a high-resolution RBG (red-blue-green) sensor, and lidar payload were used to refine the surface model of Enoch in the flooded area using Agisoft Metashape Pro. The bare earth sur- face model was reviewed to validate point cloud clas- sification results aligned with ground-benchmark data. Remaining data artifacts were then manually removed. Watersheds that flowed into or out of the Enoch study area were delineated using the ESRI (Environmental Systems Research Institute, Inc.) ArcGIS Pro water re- source management hydrologic scripting routines. A hydro-flattened DEM was created and mosaiced with existing UGRC DEM data. Manual DEM recondition- ing was performed to establish natural surface flow 100 Snow Drought and Monsoon Floods—Hydrological Extremes in the Cedar Valley Watershed During Water Year 2021, Southwestern Utah Mueller, E.R., Sudweeks, G.P., Ashston, S.A., and Olson, M.C. Geology of the Intermountain West 2023 Volume 10 through bridges and culverts, including densely vege- tated areas. Stormwater-infrastructure data provided by the Cedar City Engineering Department included culverts, storm drains, weirs, open channels, and riser pumps, but additional data on culvert dimensions were collected in the field using high resolution Global Posi- tioning System (GPS). Pre-processed shapefiles were imported into HEC- RAS as feature layers and allocated respectively to a ge- ometry type as 2D Flow Areas, Perimeters, Breaklines, and Refinement Regions. Vector data representing structures such as culverts, bridges, weirs, and trapezoi- dal channels were characterized by their attributes need- ed to calculate hydraulic flow and dynamically linked to their catchment domain. Unsteady flow was modeled throughout the catchment network in HEC-RAS using a flow hydrograph and rating curve calculated based on peak precipitation rate and expected peak discharge lag time. Field measurements of high water using GPS and online public surveys were used as benchmarks for model calibration. Our voluntary online public survey asked participants to provide a map location, the date and time of when the flooding occurred, and depth of inundation (Ashton and others, 2022). Unsteady flow was computed iteratively to match benchmark calibra- tion parameters. Kairos Group, LLC. gave instruction and advised on flow-model construction, computation, calibration, and verification. RESULTS AND DISCUSSION Water Year 2021 and the Long-Term Hydrology of Coal Creek In a typical year for the Coal Creek watershed, 69% of the runoff volume occurs during the March-June snowmelt period, and 15% of the runoff volume oc- curs during the July-September monsoon period. The remainder (16%) occurs during the late fall and winter months primarily as baseflow. Furthermore, the bulk of the baseflow that occurs throughout the year likely owes its origin to snowmelt and other cold-season precipita- tion events (Julander and Clayton, 2018). As such, the total runoff volume for all periods is well predicted by the maximum SWE values at SNOTEL sites surround- ing and within the basin; 2021 had the fourth lowest to- tal runoff on record (figure 3). Similarly, the maximum SWE value is also a good predictor of the maximum snowmelt flood each year (figure 3). The influence of ENSO on winter snowpack and resultant streamflow is also evident. For both the max- imum SWE and the March–June streamflow volume, values are greater during El Nino years compared to La Nina years (student’s T-test, p<0.05) (figure 4). Howev- er, La Nina years were not significantly different than Neutral years in terms of snowpack or runoff volume (figure 4). For El Nino years, the maximum SWE was significantly greater than during Neutral years (stu- dent’s T-test, p<0.05), but the March–June streamflow was not. On average, the March–June streamflow vol- ume was about 65% greater than the median value during El Nino years, but only 8% less than the median value during La Nina years. The wettest, and two driest, years were both during Neutral conditions. In 2021, the snowmelt runoff was the lowest ever recorded during a La Nina year. Thus, overall, ENSO has a discernable effect on the watershed hydrology, tending to result in greater streamflow and snowpack during El Nino years. While the opposite is true for La Nina years, La Nina is not as strongly distinguished from Neutral conditions in terms of hydrologic variability. Figure 5 shows the hydrograph of Coal Creek and corresponding soil moisture conditions for water years 2019 through 2021, contrasting a range of conditions from high (2019) to average (2020) snowmelt with min- imal monsoon activity, to that of 2021 with very low snowmelt and frequent monsoon floods. Climatologi- cally, 2019 was characterized by Neutral ENSO condi- tions, whereas 2020 was a weak El Nino, and 2021 was a weak La Nina. Perhaps the weak La Nina conditions contributed to low snowpack and snowmelt in 2021, but low soil moisture conditions likely also contribut- ed to the reduced snowmelt runoff (Clayton and others, 2021). The exceedance probability for the maximum SWE at the SNOTEL sites ranged from 0.79 to 0.9 in 2021, meaning that there is a 79% to 90% chance of those values being exceeded in a given year. Yet the snowmelt runoff volume had an exceedance probability of 0.95 to 0.97, indicating an exceptionally low snowmelt run- off—the third lowest on record. Figure 5B shows that 101 Snow Drought and Monsoon Floods—Hydrological Extremes in the Cedar Valley Watershed During Water Year 2021, Southwestern Utah Mueller, E.R., Sudweeks, G.P., Ashston, S.A., and Olson, M.C. Geology of the Intermountain West 2023 Volume 10 Figure 3. (A) Runoff volume for total annual runoff, snowmelt period runoff, and monsoon period runoff versus the maxi- mum snow water equivalent (SWE) as measured at the Midway SNOTEL site. Values for 2021 are shown with an X through the symbol. (B) Peak snowmelt discharge in cubic feet per second (cfs) of Coal Creek versus the maximum SWE as measured at the Midway SNOTEL site. 2021 shown in red. Figure 4. (A) Box plots showing the variation in maximum snow water equivalent (SWE) for four sites bounding the Coal Creek watershed and (B) box plots showing the variation in March through June total stream flow for Coal Creek, differen- tiated by El Nino, Neutral, and La Nina years. Boxes represent the median and interquartile range (IQR), mean values are indicated by an x, and outliers are shown as open circles (identified as values more than 1.5 times the interquartile range above the 3rd quartile or less than 1.5 times the interquartile range below the 1st quartile). 102 Snow Drought and Monsoon Floods—Hydrological Extremes in the Cedar Valley Watershed During Water Year 2021, Southwestern Utah Mueller, E.R., Sudweeks, G.P., Ashston, S.A., and Olson, M.C. Geology of the Intermountain West 2023 Volume 10 soil moisture at SNOTEL sites never reached the levels attained in 2019 and 2020, suggesting that the soils at depth were not fully saturated. This would reduce the amount of direct surface runoff from snowmelt as wa- ter continued to be stored in soils (Clayton and others, 2021). Figure 5B also shows significant increases in soil moisture following monsoon rain events such that soil moisture at the end of the water year was higher than in 2019 and 2020. More than 10 monsoon floods of Coal Creek exceeded the snowmelt flood during 2021 (figure 5C), and the proportion of annual runoff that occurred during the monsoon period (July 1 through September 30) was the greatest in the observational record and nearly equivalent to that of the snowmelt period (March 1 through June 30) (figure 6). Total annual flow in 2021 was about 10,250 acre-feet, 42% of the long-term aver- age and the fourth lowest recorded. Annual floods of Coal Creek are caused by either snowmelt, as in 2019, or monsoon rainfall, as in 2020 and 2021 (figure 5A). Figure 7 shows the average day of year (Julian day) for the peak flood of each water year dating back to 1935. For the 86-year record, snow- melt caused the peak flood 35 times (41%), and rainfall caused the peak flood 51 times (59%). The data suggest something of a cyclicity in the dominant mode produc- ing the annual flood, which may reflect longer timescale Figure 5. (A) Hydrograph of Coal Creek for water years 2019 through 2021. Gray shading indicates snowmelt period, and yellow shading indicates monsoon period. (B) Soil moisture at 8 and 20 inches depth using the mean value from six SNOTEL sites bounding the Coal Creek watershed. Soil moisture plot shows mean and stan- dard deviation for the six sites on October 1 and March 1 of each year. (C) Hydro- graph of Coal Creek for water year 2021. 103 Snow Drought and Monsoon Floods—Hydrological Extremes in the Cedar Valley Watershed During Water Year 2021, Southwestern Utah Mueller, E.R., Sudweeks, G.P., Ashston, S.A., and Olson, M.C. Geology of the Intermountain West 2023 Volume 10 climate controls on either winter snow accumulation or monsoon activity (Waite and Thomas, 1955; Hereford and Webb, 1992; Nowak and others, 2012). In the last twenty years, 15 of the largest annual floods were pro- duced during monsoon rainfall (figure 7). Figure 8 shows the USGS standard Log-Pearson Type III flood-frequency analysis for the annual snow- melt floods, annual rainfall flood, and the annual flood regardless of when it occurred (combined snowmelt and rainfall flood populations). As expected, the peak magnitude of snowmelt floods is roughly half or less than that of rainfall floods for a given return interval. The largest recorded snowmelt flood with an approxi- mate 100-year recurrence was 1820 cubic feet per sec- ond (cfs), which would be approximately a 5-year recur- rence for rainfall floods. In 2021, the peak flood during the monsoon season was 2000 cfs. While a relatively common flood with a recurrence of 6 to 7 years, the re- peated flooding during this event caused considerable localized flooding as discussed in the next section. It is clear that for the combined record the highest discharg- es with recurrence intervals greater than about 5 years occur primarily due to rainfall (figure 8). On the other hand, smaller floods with recurrence intervals less than 5 years may be caused by either mechanism in the com- bined record. Treating the flood frequencies separately based on the different flood-generating mechanisms (snowmelt versus rain) demonstrates that published flood probabilities combining flood types may underes- timate the magnitude of rare (100-year) monsoon rain- fall floods (Yu and others, 2022) (figure 8). Precipitation Extremes and Monsoon Floods In the context of the above discussion of watershed hydrology for Coal Creek, water year 2021 can viewed as a year of contrasts – hydrological (snow) drought overprinted by an intense monsoon season that resulted in numerous floods in Cedar Valley. Point precipitation measurements during the three floods analyzed herein all showed locations where precipitation rates exceeded the computed 100-year precipitation rates for durations between 1 and 24 hours. In the following subsections, we describe the precipitation climatology relative to the depth-duration frequencies published by NOAA (Bon- nin and others, 2011), and, where available, streamflow hydrology, for the three major monsoon floods that im- pacted Cedar Valley in 2021. Cedar City, July 26th, 2021 The storm of July 26th produced intense precipita- tion over much of Cedar City. Most of this precipitation fell over an urban area with rapid overland flow along roadways, causing significant flooding of residenc- es and roadways in low areas near the intersection of Figure 6. Proportion of total annual runoff derived during the snowmelt (March 1 through June 30) versus monsoon (July 1 through September 30) periods plotted versus the an- nual flow volume for Coal Creek. Figure 7. Julian day of annual flood peaks from 1935 to 2021, with the 10-year moving average centered on each point shown as a solid line. Peaks prior to June 15 are considered to be derived from snowmelt, and after June 15 from mon- soon rainfall. 104 Snow Drought and Monsoon Floods—Hydrological Extremes in the Cedar Valley Watershed During Water Year 2021, Southwestern Utah Mueller, E.R., Sudweeks, G.P., Ashston, S.A., and Olson, M.C. Geology of the Intermountain West 2023 Volume 10 Interstate 15 (I-15) and West 200 North. Precipitation rates exceeded the 200-year event for one of the gauges for durations of 30 minutes to 3 hours (figure 9A). At this gauge, more than 2.5 inches of rain fell in 1 hour. The National Weather Service gauge recorded more than 2 inches of rain in approximately 1 hour, which is greater than a 100-year event. Several other gauges recorded between 1.5 and 2.5 inches of rain in 2 hours (figure 9A). City officials declared a State of Emergency following this event due to the extensive damage caused by flooding of homes in the area. Enoch, August 1st, 2021 Precipitation rates during the August 1st flooding in Enoch were somewhat less than measured less than a week earlier in Cedar City. One gauge showed precip- itation rates that exceeded the 100-year event for du- rations of 2 and 3 hours, with total accumulations of 2 to 2.5 inches (figure 9B). Yet, no precipitation gauges exist just east of and in the vicinity of I-15, where sev- eral small and steep drainages provide direct inflow to low-lying urban areas already experiencing heavy rain. Radar data during the event shows high precipitation rates across these drainages and directly over the south- ern part of the city for nearly an hour (figure 10). There is also clear evidence of flooding from these drainages in the form of debris, high water marks, and fresh sed- iment movement. The water from the drainages passed through culverts beneath or overtopped I-15, temporar- ily closing the roadway. The floodwaters then inundated low-lying areas and neighborhoods of Enoch, causing extensive damage. HEC-RAS modeling of flood inundation provides a clearer picture of the flood extent on August 1st (figure 11). Using a discharge hydrograph calibrated to mea- sured high-water marks in tributary basins and flooded areas, the modeled inundation shows significant flood- ing of neighborhoods in low-lying areas of Enoch. The modeled depths and extents are also consistent with feedback from public surveys on flood damage and ex- tent at individual properties (figure 11D). The extent of flooding on August 1st caught many residents of Enoch off guard because there are no major drainages in the area. Because Cedar Valley is a closed basin, floodwa- ters emanating from steep tributaries along the range front do not necessarily coalesce into a primary channel with a higher conveyance, but instead flood across large areas at relatively shallow depths (figure 11). This high- lights the fact that small drainages can produce large floods during convective monsoon storms, and flood- ing may be exacerbated by rapid runoff from urbanized areas. Coal Creek, August 18-19, 2021 Precipitation rates during the Coal Creek floods ex- ceeded the 500-year event for one gauge, and the 100- year event for four gauges, but over longer durations of 6 to 48 hours compared to the floods described above Figure 8. Log-Pearson Type III fits to annu- al peak flood flows derived from snowmelt, monsoon rain, and the combined snowmelt and rain periods as measured at the Coal Creek USGS stream gage from 1935–2021. 105 Snow Drought and Monsoon Floods—Hydrological Extremes in the Cedar Valley Watershed During Water Year 2021, Southwestern Utah Mueller, E.R., Sudweeks, G.P., Ashston, S.A., and Olson, M.C. Geology of the Intermountain West 2023 Volume 10 (figure 12). The highest precipitation rates were record- ed near the rim of Cedar Breaks at the headwaters of Coal Creek, where 5 to 6 inches of rain fell in 12 hours and storm totals exceeded 6 to 7 inches at two gauges in this study. Cedar Breaks National Monument mea- sured over 6 inches of rain according to media reports. These are among the highest 24-hour rainfall totals ever recorded in Utah – 6.78 inches at the U.S. Highway 14 MesoWest site, and 6.1 inches at the Midway Valley SNOTEL site (figure 12C). According to NOAA’s State Climate Extremes Committee, the 24-hour precipi- tation record in Utah is 5.08 inches; but higher values have been reported in media stories at regional weather stations not likely considered by NOAA. The initial pulse of intense precipitation began around midnight on August 18th, and caused a rap- Figure 9. Depth-duration-fre- quency precipitation plots during (A) the July 26th flooding of Ce- dar City, Utah, and (B) the Au- gust 1st flooding of Enoch, Utah. Symbols represent data from different Weather Underground personal weather stations, ex- cept for the Cedar City Nation- al Weather Service (NWS) site shown as a diamond on A. 106 Snow Drought and Monsoon Floods—Hydrological Extremes in the Cedar Valley Watershed During Water Year 2021, Southwestern Utah Mueller, E.R., Sudweeks, G.P., Ashston, S.A., and Olson, M.C. Geology of the Intermountain West 2023 Volume 10 id rise in Coal Creek to a peak discharge of 2000 cfs at around 3 a.m. (figure 13). This was the largest flood peak of the water year. This initial flood pulse was fol- lowed by a second, smaller pulse of 1310 cfs. After a period of quiescence, a second storm impacted the wa- tershed near midnight on August 19th with as much of 2 inches of additional precipitation focused more in the center of the watershed. This caused a third flood pulse of approximately 1000 cfs (figure 13). Residents reported all three flood pulses, and sig- nificant flooding occurred along a channel of Coal Creek north of the Cedar City airport. Cedar City has Figure 10. NOAA radar reflectivity mosaic (in decibels, dBZ) showing the August 1, 2021, storm that caused flooding in Enoch, Utah. Top panel shows the 1-hour build up to the peak rainfall rates, middle panel shows 15 minutes during the most intense part of the storm, and lower panel shows the final half hour and initial dissipation of the storm. 107 Snow Drought and Monsoon Floods—Hydrological Extremes in the Cedar Valley Watershed During Water Year 2021, Southwestern Utah Mueller, E.R., Sudweeks, G.P., Ashston, S.A., and Olson, M.C. Geology of the Intermountain West 2023 Volume 10 built flood diversion structures, channels, and levees to protect against flooding of Coal Creek, but some of this infrastructure was overwhelmed by the high debris (wood) and sediment loads of the flood. The intense precipitation during the initial flood event occurred in steep terrain that likely resulted in debris flows and overland flow that picked up woody debris and trees from the forested hillslopes. The debris and sediment clogged diversion structures and channels, resulting in levee overtopping and flooding of some residential areas. Whereas, this was not a particularly large flood (recurrence of 6 to 7 years), it demonstrates the import- ant role of sediment and debris loads in exacerbating flood effects – especially in communities at the mouths of steep mountain watersheds where monsoon floods may occur. CONCLUSIONS Water year 2021 in southern Utah was character- ized by an extremely muted spring and early summer snowmelt period that worsened existing drought condi- tions, followed by a strong monsoon season that caused Figure 11. HEC-RAS model results showing inundation depth (in meters) from (A) flood initiation to (B) maximum pre- cipitation to (C) maximum flood extent during the August 1st Enoch, Utah, flooding. (D) Results of public survey showing a subset of impacted homes and estimated inundation depth; note that map in D is not at the same scale as A through C, as indicated by the black lines showing common points in each image. 108 Snow Drought and Monsoon Floods—Hydrological Extremes in the Cedar Valley Watershed During Water Year 2021, Southwestern Utah Mueller, E.R., Sudweeks, G.P., Ashston, S.A., and Olson, M.C. Geology of the Intermountain West 2023 Volume 10 Figure 12. Depth-duration-frequency pre- cipitation plots for the August 18th and 19th flooding of Coal Creek, showing (A) low-ele- vation sites, (B) a mid-elevation site, and (C) upper-elevation sites. 109 Snow Drought and Monsoon Floods—Hydrological Extremes in the Cedar Valley Watershed During Water Year 2021, Southwestern Utah Mueller, E.R., Sudweeks, G.P., Ashston, S.A., and Olson, M.C. Geology of the Intermountain West 2023 Volume 10 numerous floods. The runoff of Coal Creek is vital to the water supply of Cedar City and the groundwater re- charge of the Cedar Valley’s aquifers. Here we show that the overall streamflow is predictably related to snow accumulation and different phases of ENSO. In 2021, low snowpack and soil moisture during a weak La Nina contributed to extremely low snowmelt runoff. The dry conditions were followed by an unusually intense mon- soon season. Measured precipitation rates exceeded the 100-year rainfall rates, and in some cases the 500-year rainfall rates, for multiple storms during the 2021 sum- mer monsoon, resulting in localized flooding through- out Cedar Valley. The floods in Cedar City and Enoch were likely exacerbated due to extreme precipitation over urban areas, and hydraulic modeling shows good agreement with measured inundation and public sur- veys of the impacted area during the August 1st flooding of Enoch. For Coal Creek, there are no long-term trends in discharge volume or flood frequency in the stream gaging record, and no individual event in the gage re- cord during 2021 broke any records. Yet the hydrology of water year 2021 was unique in the combined effects of an extremely low snowmelt followed by an intense monsoon season that resulted in widespread flooding of residential areas, an unfortunate but instructive com- bination from the perspective of water resources and hazard analysis. ACKNOWLEDGMENTS We would like to thank the Southern Utah Universi- ty Geosciences Department and GeoClub for support- ing this research. David Maxwell provided surveying and GIS resources and expertise that greatly facilitated the field research and hydraulic modeling. The com- munity of Enoch provided invaluable information on flooding extent and damage via public surveys. Jordan Clayton (NRCS) and Paul Grams (USGS) provided helpful reviews that improved the clarity of the manu- script. REFERENCES Adams, D.K., and Comrie, A.C., 1997, The North American Monsoon: Bulletin of the American Meteorological So- Figure 13. (A) Hydrograph of Coal Creek and (B) accumulated precipita- tion during the August 18th and 19th flooding of Coal Creek. 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