In 1965 the Federal Government of Nigeria requested assistance from the United Nations Special Fund 1 in setting up a research team at Kainji Lake. The project was approved in 1966 and the Food and Agriculture Organization of the United Nations was named executing agency. The purpose of the project was to assist in the comprehensive development of man-made lake resources through research and surveys, the results of which would be made available to all regions of Nigeria. Within this general framework the limnological programme would carry out studies on the physical, chemical and biological changes taking place in the aquatic environment and their effect on the fish fauna and on aquatic weeds. Mr. S. G. Karlman was appointed to participate in these studies as an Associate Expert with support from the Government of Finland. He served from 12 September 1970 to 30 April 1973.
The lake can be divided into three main basins with different characteristics. The northern part where the Niger enters the lake is narrow with rather steep shores on both sides and comprises about 10% of the surface area (Table 2). Because of the steep and narrow section the water movement is rather swift, resembling riverine conditions more than lacustrine. The middle part of the lake represents about 70% of the surface area. The eastern shore is shallow, exposing a large drawdown area at low water level; the western shore has a steeper character. The depth is about 11 m in the mid basin except in the old river channel where depths of over 20 m are found. The main part of the basin covers the submerged Foge Island, which was partly cleared of vegetation before inundation. Foge Island divides the river into two channels at the northern end, and the channels merge again north of Old Bussa. The southern and deepest part of the lake represents about 20% of the surface area. The rather steep shores make it least affected by the annual drawdown. There are three larger islands extending above the maximum level, the northern peak of Foge Island and the heights at Old Bussa and Garafini. During low water large parts of Foge Island rise above the water and are temporarily inhabited by migrating fishermen.
The climate is governed by the annual cycle of humid air masses penetrating from the ocean, the monsoon, followed by a southern movement of dry air from the Sahara Desert, the harmattan. The year therefore has two distinct seasons, the wet season which begins in June and lasts until October and the dry season which lasts for the rest of the year. During the wet season, the air temperature rises to a mean maximum of about 35° C; the mean minimum is about 24° C. During the harmattan period the mean maximum is about 38° C and the mean minimum 16° C. The average annual rainfall is about 1018 mm (see Fig. 2). Strong winds occur normally only during the beginning and end of the wet season when the intertropical front passes the area. There is only one hour difference in daylength between the longest and the shortest day of the year at Ilorin's latitude (about 160 km south of Kainji).
The regime of the Niger River is determined by its peculiar geographical situation. The run-off from the rains in the Fouta Djallon mountains travels 2700 km before reaching Nigeria six months later. The water from the upper catchment area is comparatively clear when it reaches Nigeria, having deposited its silt in the swampy areas (for this reason it is known as the “Black Flood”). The second drainage area contributing to the run-off starts downstream of Niamey, where tributaries in Dahomey flow north into the Niger. In Nigeria the rivers Sokoto and Malendo and a few smaller streams contribute to the local flood above Kainji Lake. The local flood water has a milky appearance due to the silt it carries, and is therefore called the “White Flood”. The peak of the Black Flood reaches Kainji in January–February with a discharge of about 2000 m3/s falling to less than a hundred m3/s in May–June. The White Flood starts rising in July and it reaches its peak inflow in late September with a maximum discharge of about 2500 m3/s. The lake level starts to rise in late August with the incoming White Flood and reaches the maximum level in December during a normal year (Table 1 ). This level is maintained until March, when it starts to fall, reaching normal minimum in August (Fig. 3). The drawdown is about 10 m during a normal year.
1 For further details see A Limnological Description of Kainji Lake, by F. Henderson.
1.4.1 Temperature Distribution
Kainji Lake is a warm monomictic lake with a period of total mixing in December–January due to nocturnal cooling during the harmattan period. After this period the increasing temperature prevents mixing and thermal stratification develops, lasting until April–May. The thermocline develops at about 7–10 m depth and then gradually sinks until it disappears* The rapid deepening of the epilimnion during stratification is explained by the discharges from the lake as the levels of the spillways and turbine intakes are situated below the thermocline.
1.4.2 Oxygen
During the period of total mixing the oxygen concentration is nearly uniform from surface to bottom. After the lake has stratified, the oxygen concentration in the hypolimnion drops quickly, reaching zero within the first month. The deoxygenation goes so far that oxygen disappears from the hypolimnion and hydrogen sulphide is produced. The epilimnion maintains a high oxygen concentration throughout the year except during the partial stratification that may develop during the day, when the concentration decreases with depth. The surface water mostly has an oxygen content close to saturation value. The saturation is lowest in the morning and rises to 100% and above in the afternoon.
1.4.3 Nutrient Content
The Niger River has a very low mineral content and is in general poor in plant nutrients; the concentration of sulphate, phosphate, nitrogen and magnesium are particularly low (Imevbore, 1970). No detailed studies have yet been made of the chemistry of the lake water but the low conductivity, 45–60 micromhos/cm, and the low alkalinity of about 0.70 mEq/1 indicate that the flooded soil in the reservoir area has not had any significant impact on the chemistry of the lake water. The high theoretical flushing rate of the lake (four times a year) suggests that the nutrient content of the lake will not essentially differ from that of the river.
1.4.4 Turbidity
The turbidity of the local flood water is mainly caused by fine clay particles, partly of colloidal nature. A seasonal variation is caused by the interaction of the two flood regimes. Turbidity plays a significant role as it determines the transparency of the water and thus the light available for photosynthesis at various depths.
1.4.5 Plankton
The abundance and composition of both phyto-and zooplankton have been studied by the counterpart limnologist. The results of the studies will soon be published. No attempt is therefore made here to analyse the influence of the variation and composition of phytoplankton on the primary production before detailed results are available.
A large proportion of the rocks in the area consists of ancient crystalline types. A major unconformity separates these crystalline rocks from sandstones, conglomerates and gravels found overlying them in certain areas upstream of Shagunu The sediments form part of the Nupe sandstone group that was deposited in a shallow area during an earlier geological period. Limestone and other soluble sedimentary rocks are absent in the reservoir site.
1.6.1 Terrestrial Vegetation
The lake area lies on the border of the Sudan and northern Guinea savanna. The typical vegetation of this area is characterised by grasses and fire tolerant trees and shrubs. Bush fires are common in the area during the dry season, with early fires starting in November.
Before inundation, part of the Foge Island (about 4,000 ha) was cleared of vegetation to provide fishermen with a fishing ground where nets could be set without being entangled in the vegetation*
1.6.2 Aquatic Weeds
The development of aquatic weeds has been monitored since the creation of the lake. Until now no serious problems have arisen from sudd (floating plant material) formation. The most common weed in the lake Echinochloa sp, is found on sheltered shores around the lake and has also formed a sudd extending for about 10 km on the shallowest part of Foge Island north of Dugga. The area dries during low water and the weed community dries out but re-establishes itself with the rising water level in September.
A large part of the arable land is cultivated during the wet season for production of staple food crops. Investigations are under way to estimate the agricultural potential of the drawdown area and preliminary results indicate that there is a potential area of 3000–4000 ha suitable for irrigation development. There are no major centres of population in the area except New Bussa township and Yelwa. Industrial activity is planned for the New Bussa area but no development has yet taken place.
The dam comprises a 550 m long concrete gravity dam and a fill dam of over 7 km length (Table 2). It has a height of 65.5 m with a crest elevation of 145 m above normal sea level. It contains the turbine intakes at 100.6 m and the four spillway gates at 126.5 m. The initial installation is four generating sets of 80 MW capacity each. The potential installation is 12 turbines with similar capacity.
The navigation lock system comprises two locks with an intermediate basin, the upper look being incorporated in the concrete dam. The maximum difference in height between the reservoir water level and the tailwater level is 41 m. The size of the lock is 198 × 21 m with a depth of about 3 m and a volume of about 12,000 × m3.
Since the filling of the lake there hag been no treatment with insecticide of the tributaries flowing into the lake, nor is there a sewage outlet in the lake. New Bussa is the only community in the drainage area with a sewage system. The sewage water is infiltrated and evaporated after treatment. There is an increasing use of fertilizers in the area but no figures are available for the amount used per surface area. The use of synthetic fertilizers may in the long run lead to a slight eutrophication of the lake with some consequences for the oxygen balance.
Kainji Lake has the highest fishing effort in comparison with other large African lakes, with a density of fishermen of about five per square kilometre (1970).
