JOURNAL OF ENVIRONMENTAL GEOGRAPHY

Journal of Environmental Geography 8 (1–2), 1–10.

DOI: 10.1515/jengeo-2015-0001

ISSN: 2060-467X

DOWNSTREAM MORPHOLOGIC CHARACTERISTICS OF THE ALLUVIAL

SECTION OF LOWER RIVER OGUN, NIGERIA

Olutoyin A. Fashae, Adetoye Faniran

Department of Geography, University of Ibadan, 200284 Ibadan, Nigeria

*Corresponding author, e-mail: toyinafashae@yahoo.com

Research article, received 23 July 2014, accepted 9 February 2015

Abstract

Rivers constitute an important focus of attention in surface water studies because of their dynamic nature. Therefore, natural rivers de-

velop a wide range of channel forms whose characteristics vary as a function of the position within the fluvial systems. This study exam-

ined the river channel morphologic parameters along the alluvial section of River Ogun in South western Nigeria. Data on the channel

morphologic variables were collected through field measurement of the bankfull cross sectional characteristics of the river from where

the longitudinal characteristics were defined. 48 cross sections were randomly established at bankfull stage along the river channel

stretch of 90 km. Bankfull depth and width at each of the cross sections were determined using sonar (electronic sounding machine) that

was mounted to a boat. Velocity was measured with the aid of a current meter, while other morphological parameters were estimated

from the field data. Analysis of variance revealed that downstream morphological characteristics of the river varies distinctively at each

cross section with bedslope as the most significantly varied among all other morphologic parameters (F=91.18; P=0.00). Pearson product

moment correlation technique revealed that bankfull width had a correlation of 0.8 and 0.9 with wetted perimeter and cross sectional area

respectively while bankfull depth (maximum) had correlations of 0.9, 0.8 and 0.78 with hydraulic radius, wetted perimeter and cross

sectional area respectively. The research also revealed that gradient affects the discharge with a positive correlation of 0.9. The study

ascertains the extent of variability in the morphologic characteristic of River Ogun which provides scientific basis for river maintenance

and management.

Keywords: bankfull depth, bankfull width, alluvial section morphologic characteristics, river channel, Lower River Ogun

INTRODUCTION

The assessment of river condition relative to some ideal

state is a concept receiving increasing attention in fluvial

geomorphology. The morphology of any river shows a

great variability and dynamic behaviour. Therefore, the

river channel as a subsystem and an important compo-

nent of the river basin system deserves studying in some

details to enhance river-basin management which pro-

vides scientific grounding for river maintenance and

management. Monitoring of channel morphology ex-

tends understanding of types and rates of responses to

environmental changes. A proper understanding of this is

essential for mined-land reclamation, channel modifica-

tion for flood control and navigation, identification of

areas of active tectonics and the litigation of boundaries

etc. (Elliot, 1984). River channel morphology provide

information on river characteristics and behaviour, in-

fact, river morphology has been a subject of great chal-

lenge to scientists and engineers who recognized that any

effort with regard to river engineering must be based on

a proper understanding of the morphological features

involved and the responses to the imposed changes

(Chang, 2008). Examining river network behaviour en-

hances understanding of the way in which geomorphic

processes behave across networks.

Several river condition assessment methods have

been designed for countries worldwide; AusRivAS (Par-

sons et al., 2002), the Index of Stream Condition (Lad-

son and White, 1999; Ladson et al., 1999), the River

Habitat Audit (Anderson, 1993) and River Styles (Bri-

erley and Fryirs, 2005). River Styles for instance, pro-

vides a framework for assessing river condition using

geomorphic criteria and achieves this largely by compar-

ing the geomorphic character of reference reaches to test

reaches of similar river types (Brierley and Fryirs, 2005).

The investigation of river channel morphology and the

attendant features is an interesting aspect of geomor-

phology which is of immense importance in understand-

ing the processes affecting landforms. Therefore, geo-

morphic river condition assessments are valuable mech-

anisms for determining the present and future health of

river systems (Maddock, 1999).

River morphology depicts the form of a river along

its length and across its width and consequently its

shape. River morphology is explained by channel pat-

terns and channel forms, and is influenced by such fac-

tors as discharge, water surface slope, water velocity,

depth and width of the channel, amount and size of the

transported material, river bed materials, etc. These fac-

tors are not independent but inter-related to each other.

Several studies had been carried out on the form and

2 Fashae and Faniran (2015)

shape of river channels for instance, Soar et al., (2001

suggests that stream system adjusts in order to maintain

a steady state, or dynamic equilibrium between the driv-

ing mechanisms of flow and sediment flow and the re-

sisting forces of bed and bank stability and resistance to

flow. Ward and D’Ambrosio (2008) in their study on

stream classification identified factors that can influence

channel morphology and concluded that channels with

bedrock, have limited sediment supply whereas cobble

and gravel bed channels are high energy channel with

high sediment supply. Therefore, erosion instability,

mass wasting and debris flow are more dominant pro-

cesses as the bed material become finer and these affect

channel morphology. They also emphasize slope as a

major factor in channel morphology, as slope changes

from upstream to downstream, an in relation the channel

morphology also changes. Moreover, Montgomery and

Buffington, (1997) emphasized that spatial variation is

sediment supply may govern channel morphology in

different segment of rivers. Channel response to increase

sediment supply depends on the ratio of transport capaci-

ty to sediment supply. They linked the variables of chan-

nel morphology such as width, depth, bed slope grain

size, bed forms and patterns to function of sediment

supply, transport capacity and vegetation. Transport

capacity in terms of frequency, magnitude, and duration

of discharge and slope. Riparian vegetation also influ-

ences channel morphology in different ways. Vegetation

protects banks from erosion and increases flow re-

sistance by increasing roughness and reducing flow

velocities so that channels with dense riparian forests

tend to be narrower (Brookes et al., 2000). Moreover,

vegetation on river banks and woody debris within the

channel may act as sediment traps that create different

channel morphologies and modify the channel type

(Schumm, 2005). All these factors affect river channel

morphology.

Over the last several decades, stream morphology

researches has been undertaken by scientists in a wide

variety of disciplines, yet our understanding of channel

morphology, features and the factors influencing them

is still incomplete. Most geomorphological investiga-

tions involving channel morphometry are concerned

with the definition, measurement and analysis of quan-

titative indices describing the cross section, the bed-

form and long profile as well as the plan geometry of

rivers (Goudie et al., 1990). According to Goudie

(1990), morphology and particularly the cross section

and plan-form properties of the channel has increasing-

ly been linked to river flow characteristics which are

also related to properties, quantities of bed materials

and transported sediments.

Every river channel has its own characteristics

that is unique in its own way and the dynamism in-

volved in downstream river morphologic variables

suggests the need for quantitative understanding of the

behavior of river morphologic variables and this re-

mains an important but yet elusive goal in fluvial geo-

morphology. Alluvial rivers are dynamic landforms

subject to rapid change in channel shape and flow pat-

tern. Examining alluvial river network behaviour en-

hances understanding of the way in which geomorphic

processes behave across the channel. The variation in

river channel morphology is a result of great range of

hydrological conditions, sediment characteristics and

geologic histories of the river. The nature of the mate-

rials through which a river flows initiates the three

types of stream channels: bedrock, semi controlled and

alluvial. Alluvial channel is composed of sediments

transported by the river and it is susceptible to major

morphologic change and to significant shifts in channel

position as the alluvium is eroded, transported and

deposited, and as the sediment load and water discharge

changes. Since the alluvial section of any river is dy-

namic in nature in terms of its morphology, there is the

need for a quantitative understanding of alluvial cha n-

nel form and response to changes in governing condi-

tions remains an important yet elusive goal in fluvial

geomorphology (Fashae, 2011).

Analysis of river channel morphology appears to

have been largely studied as many of the research efforts

on river channels have focused almost exclusively on

channel pattern (Ebisimiju, 1994; Holz and Baker, 1981;

Beschta and Platts, 1986; Thorne, 1997; Friedman et al.,

1996). Since, river channels show some common charac-

teristics in areas of similar landform. The river channel

as a subsystem and an important component of the river

basin system deserves studying in some details to en-

hance river-basin management. This is the reason for

examining some aspects of river channel morphology

along the alluvial segment of River Ogun before empting

into the Atlantic Ocean at the Lagos lagoon by analyzing

the channel morphologic characteristics, interrelation-

ship among the morphologic variables and the down-

stream variation for channel morphologic variables.

STUDY AREA

The Ogun River basin is located between latitudes

6
o
33’N and 8

o
58’N and longitudes 2

o
40’E and 4

o
10’E

(Fig. 1). The catchment area is about 23,000km
2
.

River Ogun takes its source from the Iganran hills

at an elevation of about 530m above mean sea level and

flows southwards over a distance about 480km, before it

discharges into the Lagos lagoon. The lower River Ogun

is defined for this study as the stretch from Mokoloki

town to Isheri town downstream, especially areas under-

lain by sedimentary Abeokuta formation which consists

mostly of sandstone of medium to coarse grain, poorly

sorted and micaceous (Oyawoye, 1972). There are clay

and mudstone intercalations; cross bedding is common

and the rock is soft and friable, except where cemented

locally by ferruginous materials. The main sedimentary

rocks are the alluvial deposits, coastal plain sands both

of Quaternary age. The choice of this portion of the

Ogun River as the study site for this research work is

based on the fact that most of the principal factors that

control river geomorphology, namely: climate, geology,

hydrogeology and relief, are relatively constant along the

study segment of River Ogun coupled with the fact that

the river is perennial in its flow.

Downstream morphologic characteristics of the alluvial section of Lower River Ogun … 3

The climate of the study area is controlled by the

Inter Tropical Discontinuity (ITD). The ITD is an at-

mospheric zone between the maritime South West mon-

soon wind and the dry North East trade winds. This zone

in West Africa moves with the location of the Sun in

such a way that during the Southern summer, the ITD

moves close to the coast and during the Northern sum-

mer, the ITD moves northwards to about latitude 14-

15
o
N. As a result, the rainy and dry seasons are well

marked. The rainy season begins earlier in the south

where it lasts from March until the end of October or

early November, giving at least seven months of rainfall.

North of Oyo and Iseyin, the onset of the rains is delayed

and generally begins late in April or early May and ends

in mid-October (Ogun River Basin Development Au-

thority, 1981). In late July and early August, dry days

are prevalent and sufficiently regular to constitute what

has been termed the “little dry season”, with mean

monthly figures below 100mm. The mean wet seasonal

rainfall is about 1,015mm to 1,525mm in the Lower

Ogun river and about 510mm to 1,525mm in the Upper

(Ogun River Basin Development Authority, 1981). The

actual number of rainy days ranges from 250-280 days.

The mean annual rainfall of the study area ranges from

900mm to about 2000 mm (Ogun River Basin Develop-

ment Authority, 1981). Temperatures are fairly uniform

throughout the year with a mean annual of 26
o
C -27

o
C

with an annual range of 5
o
C to 8

o
C while the relative

humidity ranges between 60% and 80% (Ogun River

Basin Development Authority, 1981). Annual evapora-

tion rates are also high throughout the year, with month-

ly amounts varying from about 90mm in July to over

Fig. 1 Map of River Ogun showing the Alluvial River study Segment

4 Fashae and Faniran (2015)

130mm in January. The mean annual water surplus of

the study area ranges from 254mm in the North to

508mm in the coastal belt. The total annual potential

evapotranspiration is estimated at between 1600mm and

1900mm.

The Lower River Ogun is characterized by low

slope angle and extensive floodplains with marshes and

swamps. The total catchment of the lower Ogun is ap-

proximately 12,630km
2
while the length of the study

stretch is approximately 90 km. The Lower Ogun Basin,

where the study stretch lies has a mean slope of about

than 3% and the landform consists of plains with straight

and gently undulating topography. The general topogra-

phy is characterized by a low, flat plain along the river,

subject to frequent seasonal flooding and influenced by

groundwater. For instance, at Mokoloki, the river bed

sediments are coarser, making this section of the river

channel relatively shallower and narrower, water being

contained in less than 20% of the channel width while at

the much lower section around Isheri, the channel is

characterized with an average bankfull width of about

115.6m. Much as result of the downstream increase in

the discharge and it subsequent production of alluvium

that are mined for construction purposes. River Ogun

basin comprises of two major rivers; Oyan and Ofiki at

the upstream section, while other tributaries such as the

Rivers Ewekoro and Adiyan are located at the lower

extent of this study section (Fig. 1). Some of these tribu-

taries are areic with no definite course. There is a lot of

human influence on the river for instance, a number of

portable water projects were mounted on the river for the

provision of water to the rural populace. These include

the mini water works at Akute (Fig. 2).

The drainage pattern of the river is dendritic in na-

ture. Many of the subsequent and obsequent rivers and

stream often dry up completely during the dry seasons

while the consequent river (Ogun) often have reduced

water level and discharge thereby leaving extensive

floodplains and migratory bars at the sides. The lower

River Ogun is characterized by a minimum discharge

value of about 2.3m
3
/s and a maximum values of 40

m
3
/s at Akute . An increase in downstream discharge is

indicative of the sudden change in the channel gradient

due to the impoundment of water by barrage at Akute

(Fig. 2). In addition, within downstream locations, fur-

ther adjustments of the channel efficiency were achieved

by the changing channel pattern of meanders and braids.

This is evidenced with the occurrences of some fluvial

forms such as ripples and pools sequences and formation

of sand bars by the alluvium

MATERIAL AND METHODS

The morphological variables were evaluated for both the

longitudinal and cross sectional profiles of the river to

provide useful information on the state of the river chan-

nel at the time the study was carried out. The morpho-

logic variables were collected through field survey

where a total of forty-eight bankfull cross sections was

unevenly surveyed at bankfull stage along the river

channel stretch of 90 km. This is as a result of the occur-

rence of straight, meandering and braided channels along

the river channel. The bankfull width and depth were

also measured using automated SDE-28 ECHO depth

sounder (sonar) whose visual interface was mounted in

Fig. 2 Barrage at Akute along River Ogun with the insert showing the width

Downstream morphologic characteristics of the alluvial section of Lower River Ogun … 5

the boat and the traducers was attached to the base of a

moving boat, then suspended into the water to receive

sound signal that translates into the depth values. The

sonar machine has the capability of measuring the

width perpendicular to the direction of the sounding

from bank to bank. The depth values were collected at

the centerline of the river. Bedload material was col-

lected using the grab sampler technique by scooping

the river bed to trap materials for its particle size at

equidistant locations along the cross section. These

materials were mixed together to obtain a composite

representation of the cross section before taken to the

laboratory for analysis using the hydrometer method.

The coordinates of each cross section were recorded

with the aid of an attached Garmin Global positing

system (GPS). Beschta (1986) suggested that any at-

tempt at characterizing channel morphology must rec-

ognize its three-dimensional aspects, therefore all the

other channel morphologic variables, such as wetted

perimeter, hydraulic radius and cross sectional area

were calculated. Current meter was used to obtain

measurements of the flow velocity from which the

discharge of the river at each cross sectional point was

estimated. Analysis of variance (ANOVA) that states

that there is no significant variation in the downstream

morphologic variables at 0.05 significant (α-) level was

used to test for the variation of the downstream mor-

phologic variables.

RESULTS

The longitudinal profile of Lower River Ogun

The alluvial section of the Lower River Ogun channel at

bankfull stage suggests a concave-upward shape along

its downstream gradient (Fig. 3), with an elevation of

29.7m above the mean sea level at Mokoloki located at a

distance of about 10km downstream the studied segment

of River Ogun while the elevation at Isheri is 8.1m

above the mean sea level which is about 90 km down-

stream of Mokoloki, thus the river drops 21.6m along the

studied reach. Hence, longitudinal zonation of channel

forms may be recognized from the headwaters down-

stream to the river mouth.

It could be observed that there is a progressively

lower gradient and an increase in the bankfull dis-

charge as reflected the continuous addition of tributa r-

ies and increasing drainage area downstream. The

profile is punctuated at knick points where the river

cuts through valley floors as indicated at about 58km

distance which is sharply defined due to human inte r-

ference of the location of a barrage and sand mining

activity respectively. The linear relationships that

typically exists between gradient and downstream

distance along the longitudinal profile of River Og un

reveals R
2
= 0.49, a value significant at 0.05 α-level

with gradient accounting for 49% in the variation. The

significance of the gradual lowering in the channel

gradient provides explanation for the erosive and

deposition work along the meanders and braided sec-

tions of the river and the plausible reason for the i n-

crease in fluvial landforms along the study stretch.

Features such as ripples and pools sequence are evi-

denced at the concave and convex sections of these

meanders which are interspersed by braids between

the 10km to 40km downstream distance. These fe a-

tures are created by pattern scour and deposition at

bankfull discharge where the riffles tend to occur at

the inflection points and pools at bends. Also, oxbow

lake, a lake with curved plan occupying cut-off chan-

nel reach that has been abandoned were encountered

along the stretch while point bar deposits which are

sediments laid down on the inside of meander bend

largely by accretion are more pronounced.

Fig. 3 Longitudinal Profile of Lower River Ogun

6 Fashae and Faniran (2015)

number of factors are responsible for the concavity

of river longitudinal profiles, notably the energy

profiles best represented by the discharge pattern.

This is evident from the linear relationships that

typically exists between gradient and discharge

along the longitudinal profile of rivers, which in this

study reveals R
2
= 0.98, a value significant at 0.05 α-

level (Fig. 4).

Variation of River Ogun Channel Morphologic Varia-

bles with Downstream Distance

River channel morphology of an alluvial channel re-

flects the movement of water and the particle size of

the load flowing in it. The volume of water flowing

within the channel together with the sediment load

(dissolved and bedload) helps in shapening the chan-

nel morphology. A statistical description of each mo r-

phological variable along the channel is important as

it provides a general summary illustration of the

tendencies peculiar to the study (Table 1).

Depth and width are important variables of a

river channel that suggests the morphology of any

river. The maximum bankfull depth, which is indic a-

tive of the thalweg, or a line drawn to join the lowest

points along the entire length of a riverbed shows

that 4.3 m was the minimum point while the max i-

mum was 9.6 m. The range of 5.3 m in the maximum

bankfull depth indicated a wide gap between the

deepest and the shallowest points along the thalweg

from the centre line of the channel. However, the

mean bankfull depths varied from a maximum of

7.4m to a minimum of 3.7 m. Figure 3 revealed that

as the river tends towards reaching its mouth and

with increasing distance downstream, there is an

Fig. 4 Relationship between bankfull discharge and gradient along the Lower Ogun River Channel

Table 1 Morphological characteristics of the Lower River Ogun with the downstream analysis of variance (n =48)

Downstream variables Range (Min-Max)
Mean (standard

deviation)

Downstream

F-ratio

Variation (Sig-

nificance value)

Downstream distance from

Mokoloki (km)
80.8 (10.1-90.9) 25.1(16.8) - -

Maximum depth (m) 5.3 (4.3-9.6) 7.7(1.2) 0.42 0.66

Mean depth (m) 3.7 (3.7-7.4) 6.0(0.8) 1.19 0.30

Bankfull width (m) 84.4(31.1-115.6) 59.8(15.3) 0.40 0.68

Width-depth ratio (-) 14.2(5.1-19.4) 10.2(3.2) 1.21 0.31

Crossectional area (m
2
) 800.6(253.8-1054.4) 458.0(142.0) 0.08 0.93

Wetted perimeter (m) 86.3(47.6-133.9) 75.1(15.2) 0.30 0.74

Hydraulic radius 4.0(3.9-7.9) 6.0(0.8) 0.24 0.79

Discharge (m
3
/s) 29.0(10.9-39.9) 2.3(9.1) 0.67 0.8

Particle size ratio 54.0(39.2-93.2) 71.3(14.4) 0.34 0.72

Bedslope (frictional slope) 0.4(-0.1-0.3) -0.005(0.09) 91.18 0.0

Valley gradient 21.6(8.1-29.7) 17.0(6.7) 0.69 0.51

Velocity (m/s
1
) 1.0(0.2-1.2) 0.5(0.2) 0.68 0.80

Downstream morphologic characteristics of the alluvial section of Lower River Ogun … 7

increase in depth which might be as result of the

corresponding increase in downstream discharge

except for the knick points that experience a huge

increase in depth (Fig. 5). The knick points along the
studied river section are indicative of accumulation

of bedload at the barrage at Akute with an increase

in gradient. Even though depth increases down-

stream, distance alone might not account for varia-

tion pattern observed (Fig. 6).

Fig.5 Relationship between discharge and downstream distance

Fig. 6 Relationship between downstream distance and depth

The increase in depth suggests that there is a

downstream increase in suspended sediment concen-

tration which would invariably dampen turbulence

(Merritt and Wohl, 2003). It was observed that there

is a relationship between the particle size of the bed

materials (mean=71.3) and the channel form with a

decrease in size downstream. This accounts for wide,

shallow and fine/smoother banks.

Theoretically, the bankfull width of a river is a

function of its occurrence, magnitude of flow, type

of transported sediment and composition of the bed

and bank materials of the channel. The Lo wer River

Ogun channel is characterized by a minimum bank-

full width of 31.1 m, maximum bankfull width of

115.6 m and a mean of 59.76 m along the study

stretch. Although river channel widths may genera l-

ly increase downstream, a channel can still have a

stable width even though the river is migrating la t-

erally at a constant annual rate. As Clifton (1989)

suggested, overall width, depth, and cross section

area do not increase systematically downstream,

while the spatial variability results from prevailing

vegetation conditions. The width of a river can

therefore remain relatively constant where erosion

on one bank is compensated for by corresponding

sediment deposition along the opposite bank. This is

evident in some sections of the river where there are

widths changes due to their response to the fluctua-

tion in the rainfall amount and intensity that encou r-

ages the formation of migratory sand bars along this

alluvial river (Fig. 7)

However, the downstream increase in width

may be attributed to the composition of the bed and

bank materials of the river channel. T his could be

linked to removal of the riparian vegetation for ac-

tive farming activities (Fashae, 2011).

In summary, the morphologic characteristics of

the river channel vary at different points along the

river segment. This variation is indicated in the di f-

ferences in values of the variables observed from the

cross sections of the river. In order to provide an

understanding of the variability of the channel mo r-

phologic factors in the study area, investigati on of

the spatial variations of the channel form variables

along the downstream was carried out. The results

revealed that variations occurred in virtually all the

parameters downstream as reported in Table 2 where

the analysis of variance for all the channel form

variables indicated that there are variations. The F

ratio which implies the extent of variation showed

that the bed slope (F ratio 91.18 was the most varia-

ble downstream among the morphologic parameters

considered followed by width depth ratio (1.21),

while the cross-sectional area is the least variable

with an F ratio of 0.08. The bed slope with F ratio as

high as 91.18 is the most widely varied downstream,

less variations occur in particle size ratio down-

stream with F ratio of 0.34. However, o nly the varia-

tion in bed slope reflected a statistical significant

difference at 0.05 α-level while all other parameters

varying downstream albeit with no statistical vari a-

tions. This can be attributed to the fact that the study

was carried out within a definable reach that is,

along the alluvial segment (5
th

order) of River Ogun.

The significant variation in bed slope was however

attributed to the rapid changes in the work of the

river downstream, since the discharge increases sys-

tematically downstream, it is not unreasonable to

expect the down cutting of the river channel bed

changing as the flow, erosion and deposition actions

changes, even within the same reach of the river.

Furthermore, the finding that the bed slope and width

depth ratio were the most variable morphological

parameters along the 5
th

order section of the River

100

150

200

250

300

350

0 20 40 60 80 100

D
is

c
h

a
rg

e
(

m
3
/s

)

Downstream distance (km)

0 10 20 30 40 50 60 70 80 90 100

D
e
p

th
(

m
)

Downstream distance (km)

8 Fashae and Faniran (2015)

Ogun channel was instructive because the two are

about the best variables conceptually considered to

describe the changes in channel shape. The incre-

ment in width depth ratio as the river flows towards

its mouth is represented in Fig. 8.

Fig. 8 Relationship between width depth ratio and downstream

distance

Interrelationship among River Ogun channel morpho-

logic variables

The interrelationship among channel morphologic varia-

bles along the alluvial section of the River Ogun was

investigated using correlation analysis, to test the rela-

tionship occurring among the channel form variables.

This technique deals with associations between two or

more variables providing measures of the strength of

association and statistical tests of its significance. The

Pearson's product moment correlation method which

considers parametric relationships was employed be-

cause the morphologic variables were measured at ran-

dom intervals. This relationship was particularly consid-

ered with reference to the distance downstream, the

result of which is reported in Table 2.

The width depth ratio at any point along the River

Ogun channel was expectedly most influenced by the

bankfull width with a correlation of 0.9 significant at

0.05 α-level while the wetted perimeter had a correlation

of 0.8. The cross sectional area had a correlation of 0.9;

the hydraulic radius dependent on the bankfull depth

(maximum and mean) and the cross sectional area had

correlations of 0.9 and 0.8 and 0.78 respectively.

In addition, as the river channel tends to near its

mouth, the discharge, gradient and the flow velocity all

reduces in magnitude. The width depth ratio, the bank-

full width, wetted perimeter and the hydraulic radius also

changes significantly as revealed from the multivariate

graph in Figure 9. As the width depth ratio and the bank-

full width increases towards the mouth of the river, the

hydraulic radius decreases.

These were detected along the section investigated

in this study to reveal negative correlation of -0.36 and -

0.34 between downstream distance and bankfull width as

well as width depth ratio respectively, significant at 0.05

α-level occurs, while a similarly positive correlation of

0.23 occurs between downstream distance and the hy-

draulic radius. This implies that the portion of the river

distinctively describes the width, depth and hydraulic

characteristics of River Ogun channel.

Downstream of River Ogun suggests that there is

a reduction in the channel boundary resistance due to

alluvium along the banks, while the channel-bed ma-

terials become slightly fine grained. The channel bed

is composed of fine sand particles and the banks are

mainly non-cohesive (Miller, 1956). The substantial

changes observed in the correlation matrix has been

reported in other studies including pattern c hanges

0 10 20 30 40 50 60 70 80 90 100

W
id

th
-d

e
p

th
r

a
ti

o
(

Downstream distance (km)

Fig. 7 Migratory sand bars along the Lower River Ogun channel

Riparian

vegetation

Alluvium

sand bar

Downstream morphologic characteristics of the alluvial section of Lower River Ogun … 9

(Graf, 1988a), substantial widening (Burkham, 1972;

Osterkamp and Costa, 1987; Kresan, 1988) and lateral

migration (Graf, 1983b), entrenchment (Graf, 1983a)

and floodplain erosion and deposition (Wells, 1990;

Zawada and Smith, 1991).

DISCUSSION AND CONCLUSION

Rivers from source to mouth show a great variation in

morphological characteristics such that the size and

shape of the channel readily describes the section of the

river. From the study, it can be inferred that among the

eight morphological variables studied. The bankfull

width and the depth of the channel indicate the most

significant attribute of the channel form. The width

depth ratio and bed slope were found to be the most

variable morphological parameters along the studied

channel. This is instructive because the two are the best

variables conceptually considered to describe the chang-

es in channel shape. The ratio of stream channel length

to down-valley distance, which was measured on the

long profile of the River Ogun, indicated the stream type

(alluvial) and how the stream channel slope was adjusted

to that of the valley slope. The interrelationship among

channel morphologic variables along the alluvial section

of River Ogun revealed that both the width depth ratio

and the cross-sectional area at any point in the channel

Table 2 Correlation matrix of downstream channel morphologic parameters

Distance

downstream

Maximum

bankfull

depth

Mean

bankfull

depth

Bankfull

width

Width-

depth

ratio

Cross

sectional

area

Wetted

perimeter

Hydraulic

radius

Distance

downstream
1

Maximum

bankfull

depth

0.41 1

Mean

bankfull

depth

0.08 0.82 1

Bankfull

width
-0.36* -0.12 0.00 1

Width

depth

ratio

-0.34* -0.56 -0.55 0.90* 1

Cross

sectional

area

-0.10 0.46 0.48 0.82* 0.39 1

Wetted

perimeter
-0.30 0.04 0.13 0.80* 0.73* 0.90* 1

Hydraulic

radius
0.23 0.90* 0.80* 0.30 -0.22 0.78* 0.45 1

*Correlation significant at 0.05 α-level

Fig. 9 Downstream pattern of the morphologic variables along the Lower River Ogun

0.1

10 100

V
a
lu

e
s

(l
o

g
)

Distance (log)

Wetted perimeter

Hydraulic radius

Discharge

Particle (sand) ratio

Gradient

velocity

10 Fashae and Faniran (2015)

were most influenced by the bankfull width with a corre-

lation 0.82 significant at 0.05 α-level. Also, the wetted

perimeter was equally related to the bankfull width with

a correlation of 0.8 and the cross sectional area with a

correlation of 0.8, the hydraulic radius was found to be

dependent on the bankfull depth (maximum and mean)

and the cross sectional area from a correlation of (0.9

and 0.8) and 0.78 respectively. The gradient of the chan-

nel most perfectly affects the discharge with a positive

correlation of 0.9 significant at 0.05 α-level. The down-

stream increase in channel width might be due to the

loose bank materials which reflect on the bank cohesion

and roughness of the channel. The river discharge which

is closely related to the flow velocity and the channel

cross sectional area summarizes the processes occurring

within the alluvial section of the River Ogun channel and

resultant fluvial features, such as braids, incised mean-

ders, point bars, riffles and pools.

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