Lake Turkana is situated in the Great Rift Valley in the northwestern part of Kenya. Volcanic activity was frequent during the creation of the Rift Valley and lavas from the Quaternary and Tertiary ages cover much of the floor of the valley in Kenya. The lavas are mainly of alkaline type, which has important implications for the chemical composition of lakes in this area. In the Lake Turkana basin, Tertiary volcanic rocks are found in the south and along most of the western side of the lake, while a later lava flow (Pleistocene) forms a barrier in the southern end of the lake.

Quaternary sediments dominate the western and northern side of the lake. L. Turkana is in an arid and hot area. The mean annual rainfall in most of the lake surroundings is less than 250 mm. The occurrence of rainfall is very erratic and unpredictable, although the probability of rainfall is the highest during the "long rains" in March - May. The air temperature recordings at Lodwar show a seasonal pattern with the lowest temperatures in July - August, a wide range between 19.5 and 39.9 deg C, and a mean daily temperature of 29.26 deg C. The lake is exposed to frequent strong winds, the prevailing wind direction being from the southeast.

The main tributary is the Omo River, which enters the lake from the north and contributes more than 90% of the total water influx. Other rivers are temporary, flooding only during sporadic rains. The second largest river, Turkwel River, is now being dammed for hydroelectric power generation at Turkwel Gorge ca. 150 km west of the lake.

L. Turkana has no outlet, and water is lost from the lake mainly by evaporation. The evaporation rate has been estimated at 2,335 mm yr-1. The water level of this closed basin lake is determined by the balance between the influx from rivers and groundwater and the evaporation from the lake surface. Therefore the level is sensitive to climatic variations, and subject to marked seasonal fluctuations as well as to long-term periodical changes.

The development of phytoplankton is limited by the availability of nitrate and light. Light limitation was caused by turbid water and vertical mixing. The sustainable yield of traditionally exploited fish from open lake was estimated to be 15,000 30,000 t yr-1 (1).