The physico-chemical conditions of Turkwel Gorge Reservoir, a new man made lake in Northern Kenya

Abstract

Variations in some physical, chemical, and nutrient conditions were investigated at Turkwel Gorge Reservoir and its inflowing river, Suam between 1994 and 1995. Seasonal changes in inflow volume had the greatest impact on the reservoir and river conditions investigated. A wide fluctuation in inflow volume combined with a regulated outflow independent of season resulted in a draw down of over 10 m in each year. Flood inflows during the wet season resulted in the lowest values of Secchi depth (range, 0.09–2.16 m), electrical conductivity (EC, range = 140–200 mS cm −1) and total alkalinity (TA, range = 75–111 mg l −1) while the highest values were measured during the dry season. A functional relation between EC and TA (TA = 0.529 mg l −1, EC: R 2 = 0.876) suggests a predominance of carbonates among the anions. Vertical profiles of temperature and dissolved oxygen (DO) revealed that the reservoir is monomictic with a wide variation in the depth of the daily mixed layer. High values of pH (range = 6.7–8.9) and DO (range = 4.9–9.2 mg l −1) were associated with periods of peak phytoplankton photosynthesis while the lowest values followed reservoir mixing. Peak total nitrogen (TN, range = 119–526 μg l −1) and total phosphorus (TP, range = 8.9–71.6 μg l −1) levels during the wet season resulted from increased river loading. Values of dissolved reactive silica (DRS, range = 0.41–9.77 mg l −1) showed a wet season decline which was related to diatom depletion during the wet season. Annual reservoir areal loading rates of 27.38, 10.90 and 408.5 mg m −2 were computed for TN, TP and DRS respectively based on estimates of inflowing river loads in 1994. At the inflowing river Suam, low levels of EC (range = 107–210 μS cm −1) and TA (range = 62–125 mg l −1) occurred during the wet season while the highest levels occurred shortly before the river dried up. The first flood water at the resumption of river inflow in March was characterized by very low levels of DO (range = 1.8–8.2 mg l −1) and high levels of TN (range = 205–3354 μg l −1) and TP (102–1259 μg l −1). River pII (6.9–7.7) and DRs (range = 9.01–19.93 mg l −1) varied irregularly throughout the year.