The lake is located between 9° 51'N to 10° 57'N and 4° 20'E to 4° 50'E in Northwestern Nigeria (Fig. 1). The western shore-line is situated in Kwara State, the eastern one in Midwestern State. The length of the lake is about 130 km and its maximum width is 24 km. The draw-down area is exposed between maximum lake level at about 465 feet and the lowest one at approximately 435 feet under normal conditions (9.3 m difference).
The atmospheric climate is described as hot equatorial (FAO, 1965) with high maximum temperatures and an average daily maximum of 33.5° C in the warmest month. This climatic type is subdivided into the moist-dry monsoon in which the annual rainfall is between 44–100 percent of the annual evapo-transpiration.
A further distinction can be made between the northern part and southern part of the lake area, a division at approximately 10 20'N latitude (Shagunu - Gafara). These variations are caused by the movement of the “Intertropical Discontinuity” (ITD), which forms the separation between the dry continental trade winds and moist maritime equatorial air.
The northern half is characterized by four humid months and is considered a transition to the semi-arid equatorial tropical climate. The climate of the southern part has five humid months.
Representative stations for the areas are Yelwa and Mokwa respectively.
Climatological data for these stations are given by Roder (1970) and Henderson (1973).
Summarized data for Yelwa (FAO, 1965) are:
| 1. | Average daily min. of the coldest month | 14.1° c | ||
| 2. | " " max. of the warmest " | 39.1° C | ||
| 3. | Mean annual rainfall | 1010 mm | ||
| 4. | Potential evapo-transpiration | 2310 mm | ||
| 5. | Humid season: June/July | - | end September | |
| 6. | Dry season: November | - | end April | |
| 7. | Rainfall surplus (P - Ept) | during humid season is 385 mm | ||
| 8. | Draught stress (Ept - P) | during not humid season is 2185 mm |
The differences between the N and S part are evident in the rainfall figures 44 in versus 40 in the length of the rainy season (month with 1 or more inch of rainfall) - six versus seven, while in addition there is a tendency in the southern part that the rainfall distribution shows two maxima, one in June and one in September, while Yelwa station has a one peak rainfall distribution pattern (Pig. 2). Variability and distribution of the rainfall is considerable and increasing towards the North.
The humidity is related to the movement of the ITD. Highest values are recorded during the rainy season (about 80 percent). Lowest values occur in January (approx. 30 percent).
The mean annual solar radiation is 500 cal/cm2 /day.
The prevailing wind from April to October is southerly, from November to March North-NW winds are dominant.
During the rainy season squalls are common blowing from the east. They may reach peak velocities of about 45 m.p.h., occasionally 70 m.p.h. These winds induce waves of 50 – 75 cm height, sometimes even of 1 m during thunder storms.
About the change in local climate due to the formation of the lake, little is known. There are indications however that the relative humidity is less variable at Kainji than at Mokwa or Yelwa. This shows higher humidity values in the dry season at Kainji (Henderson, 1973). On its turn it promotes the formation of dew in the draw-down areas during the rainless months.
In terms of moisture and temperature the soil climate is defined as follows (SSS, 1970):
soil moisture regime - Ustic (= moisture
present during season that soil is warmest)
soil temperature regime - Ischyperthermic (= mean annual soil temperature is 22 C or more)
Overall climatic conditions are suitable for one crop per annum, soil temperatures are optimum for plant growth but extreme high temperatures in April and May may have an adverse effect on germination and maturing.
The soil parent material is derived from a number of geological formations (Fig. 1)t
According to NEDECO et al. (1961) the original rooks of the Basement-Complex have been subject to intense metamorphiam, while at a later stage intermediate to basic Magma intrusions caused a considerable change in rook composition. Three main lithological groups are recognised (Allum, 1961):
metasediments;
undifferentiated Basement-complex;
granites.
The composition of these metamorphosed sedimentary rooks is highly variable and include schists, gneisses, quartzites as well as phyllite. Consequently, he mineral composition is not constant; biotite, muscovite, epidote, hornblende and albite are some other major components. The general strike of the rooks is WNE and the dip is steep, normally (55 ).
The undifferentiated Basement-complex mainly comprises gneiss (also mapped as gneissose B-C), but quartzite and schists may also occur (Klinkenberg, 1965). A larger part of this group classifies as banded granitic gneisses and are believed to have been formed by granitization of pre-existing metasediments (Allum, 1961). The most common minerals are quarts, felspars (mainly N-plagic-class), biotite and hornblende (Pullan et al, 1964).
The “granite” is mapped as autochtone rooks and as intrusions. It includes undifferentiated granites, granodiorites and probably occasional seyenites. Porphyritic granite is common in intrusions and often contains large phenocrysts of microcline (K-felspar).
Sedimentary rooks of the Nupe Formation are exposed from Shagunu to Papiri. Fine grained standstone is the major rook type, although silt-stones and conglomerates do occur also. This formation overlies the regionally metamorphosed rocks of the B-C. There are indications that the strike is approximately NNW and the dip 15° to the ENE.
This formation is of limited importance as it is not widely occurring. It is mainly exposed between Shagunu and 10 km northwards along the shore.
Alluvial deposits are common along the streams and former tributaries of the Niger river; in addition to a large area between Warra and Gafara. Loamy and clayey textures are dominant.
The various rook types may play a decisive role in the soils derived from them and determine to a large extent their physical and chemical properties. No data are available on the elemental composition of the rooks. The change in the soil parent material and in the soils themselves due to impoundment are discussed in Chapter 2.
The underlying geology controls to a large extent the topographical features of the upland areas (Allum, 1961) as well as the shore-line (Halstead, 1971).
On its turn the slope of the land determines the acreage of the draw-down area. It is for this reason that a theoretical approach to the calculation of the acreage of the drawdown area fails.
In a qualitative survey Halstead (1971) describes three schematic shore profiles: 1) high relief-rocky coast; 2) medium relief-gravel shore and 3) low relief-alluvial flats.
The following correlation with 2.3 can be made relative to the above distinction. High relief is found where the hard (silica) rocks of the B-C. and cretaceous are exposed along the lake. This situation is encountered in the major part of the western central Lake Basin as well as its NW shore and along many parts of the northern branch. It also occurs frequently in the southern lake branch from Bussa till Kainji Dam.
Areas with low and medium relief are confined to small bays and inlets and along former tributaries of the Niger river. In general softer argillaceous metasediments mark areas with low relief. One such area is located between the River Doro and the River Menai; another one occurs at the bay at the eastern side of the earth dam.
Areas with medium relief are confined to places where rooks of the Nupe Formation or pebble beds are exposed. This occurs mainly from Shagunu to Yumu.
Alluvial flats connote areas with low relief. Most important is the Gafara alluvium (Halstead, 1971) from Gafara to Wara. Other areas occur at the conjunction of rivers into the lake such as the Malendo and Wata rivers in the northern lake part and in the SW part of the lake in combination with the occurrence of metasediments (R. Menai, R. Temo and R. Doro alluvium).
The shape of the coastline is subject to constant changes because of erosion caused by wave action generated by surface winds.
This therefore anticipated that those parts of the shore exposed to southerly winds will increase in relief and subsequently will contribute little if anything to an increase in the draw-down area suitable for agricultural use.
The original vegation of the present lake draw-down area ceased to exist after the first flooding (Lelek, 1972).
In many areas tree stumps can still be observed notably along the Western shore-line and from Gafara to Agwarra. The dense formerly riverine vegetation along the Niger river channels was cleared to a large extent, leaving especially Foge Islands treeless (Hunting Surveys Ltd., 1959). The rehabilitation of vegetation consists mainly of grasses capable of withstanding the drastic changes in lake-water level during the season. They belong mainly to Echinocholera Pyramidalis and related species of grasses, such as Vossia cuspidata. They are characterised by floating stems and broad long lanceolate leaf blades. Unfortunately, no precise records on the present flora of the draw-down areas is available.
It is recommended that such studies commence as soon as possible as the area may prove suitable for a number of agricultural disciplines.
Cattle have been seen gracing the grasses on the draw-down areas and further research on the grass population in relation to ranching is feasible.
In addition the fish population of shallow (0–50 cm) flooded grassland is very high, especially of snail Tilapia. Additional studies on the possibility of fish breeding grounds in connexion with grassy draw-down areas is warranted. These areas also facilitate certain net handling operations.
Luckily there has been no report so far upon the presence of the water fern Salvinia auriculata, the water hyacinth Eichhornia crassipes nor the water lettuce Pistia stratiotes L. They have proved a pest in many (sub)-tropical lakes (Low, McConnell, 1966).
The variation in the lake water level is controlled by:
The permissible higher and lower levels are determined by National Electric Power Authority (NEPA).
The normal high water level mark (465.0 feet) is reached in January, from then onwards the water-level drops regularly.
The lowest level is determined by a prognosis of the incoming flow, normally set at the 435.0 feet contour mark. However, in 1973, this level was kept at 445.3 feet reached 11 August (Fig. 3). No further drop in the lake water volume was allowed to compensate for reduced inflow due to poor rains in the upper catchment of the Niger river. For navigational purposes two spill-gates were partly opened on 20 November 1973. As a result of the “high” low-water level, the exposed draw-down area was reduced considerably while in addition farmers were not warned about the flood-regime, resulting in loss of drops in the lower part of the draw-down area due to flooding in September–October. From August onwards the water rises quickly to 455 feet reached at the end of September, whereafter the water level rises more slowly to the maximum height reached in January/February.
The lowest part of the draw-down area, between the 443 (435) - 455 feet contour is thus exposed for a relatively short period, two months, if exposed at all.
For this reason the draw-down area suitable for cultivation is usually given from 455 – 465 feet, totalling 46 714 acres (18 905 ha).
The draw-down area is thus exposed to the following hydrological regimes:
| Rain; | May–October (rain on the areas as well at runoff and drainage from upland areas) | |
| Drop in lake-level | (draw-down): February–August (water retention by draw-down areas and drainage from upland areas) | |
| Rise in lake-level | (draw-up) : August–November (white flood) December–March (black flood) | |
| Irrigation: | Whole year round. | |
The above combination of water resources Available to the draw-down areas offers a wide scope for agricultural activities, which were not realised at the planning stage of the dam or shortly thereafter.
There is no permanent groundwater table that may encourage plant growth during the dry Months in the upland areas adjacent to the draw-down area. During the rainy season a perched water table may develop, more pronounced in the lower topographical sites.
The rate of rise in the water level of the lake corresponds variably with a similar rise of the groundwater in the soils of the draw-down areas, especially where their texture is elayey. Experiments carried out at a site east of Kainji Dam indicate that on almost flat land (slope 1 ) and lake water at the surface at 9 m of the observation mark the groundwater level was recorded at 88 cm depth. The front of the saturated soil material sloped down approximately 10 percent. Seepage of lake water and subsequent recharge of surrounding areas is thought insignificant (NEDECO et al, 1961, vol. 2) in most of the lake-shore areas. It may occur to a limited extent at that part of the shore where rocks of the Nupe Formation are exposed. However these are on their turn surrounded by impermeable rooks of the Basement-Complex. In addition recharge of the lake from the Nupe Formation during low lake water levels is thought feasible.
The influence of the lake water above the 465 contour is noteworthy. Data (Fig. 5) collected at Papiri from groundwater observation posts at 465 and 474 ft contour are not complete and should be regarded with caution. An impression of the correlation between the lake-water and the groundwater is visualised in Fig. 5.
In general a) the groundwater level is higher at 474 contour than at 465, b) the groundwater level at 465 follows more closely the changing lake-water level, c) variations in water level at 465 contour appear to be greater than at 474 contour.
