Some aspects of hydrochemistry, lake level and vegetation dynamics in the lake Elementeita basin, Rift valley, Kenya

Abstract

Some aspects of water chemistry, vegetation structure and water level fluctuations of the Lake Elementeita drainage basin in Kenya's Rift Valley were studied. The analysis of water samples showed that the lake was highly alkaline and saline. Total alkalinity and electrical conductivity of 3832 mg CaCOs/land 12842 J..lS/cmrespectively in the lake was 60-70 times higher than in rivers of the catchment and resembled conditions of other saline lakes in the Rift Valley notably: Nakuru, Bogoria and Magadi. In the vegetation analysis, mappingwas implemented from transect and profile data. The Shannon-Weiner index together with arbitrary cover abundance classes were used to estimate vegetation diversi ty and abWldance. Over150different plant species were identified within the basin in nine major plant associations. Dominantspecies in the catchment included Olea BIz-leana, JlI11peruB ~, Acacia tartlllB and TarchananthuB~tua The lake region exhibited some distinct plant communities notably the Acacia zanthophloea woodlands, T.Jr.pha-~ swamps and Sporobolus spicatus grasslands. The distribution of species around the lake was influenced by water chemistry. Cyperrm PBPYrus for example was restricted to areas where fluoride concentration was below 1.0 mall. This species was replaced by Cyperus laevigatus and Cyperrm.i...sDBUB in areas where conducti vi ty exceeded 150J,JS/cm. Climatic variations were found to be a major factor influencing water level f luct.uat.ione at Lake Elementeita. Average annual rainfall has declined in the recent past, rarely exceeding 900 mmwhile average annual temperature appears to have risen from 140Cin the 1960s to 160C in the 1980s. Apart from the influence of climatic cycles, rapid population growth in the watershed has instigated significant landscape modifications. Colour Infra-red LANDSAT imagerywas used to estimate the rate of forest and woodlanddepletion in the watershed which was found to be between 11km2/yr (1973-76) and 4 km2/yr (1976-84). mostly through conversion to agro-ecosystems. Several conclusions were drawn from findings in this study. Lakewater chemistry was found to be influenced by geology and thermal springs in the region. Volcanic rocks in the area contribute to high mineralization particularly with regard to sodium(Na+),potassium (K+)and fluoride (F-) ions. Riverkariandus was found in this regard to form a major input of mineral salts into the lake. The lake-bed springs south of the lake were also found to be a prominent source of sodium(Na+),chloride (Ol") and sulphates (S04--) ions into the lake. The loss of forest andwoodlands in the drainage basin was found to have been particularly rapid in the -1970s as a result of increased human settlements. The study concluded that such depletion will result in declines in water levels at Lake Elementeita particularly during periods of drought. Waterlevel fluctuations were however. found to emanate mainly from changes in climatic factors. The study recommends conservation of remaining tracts of Olea- Juniperus-Acacia forest and woodlands as essential ecosystems in the watershed and rehabilitation of degraded areas through improved land managementpractices. Several other recommendations have been advanced mainlyfor further research and experiment.