Impacts of land-use changes on sediment yields and water quality within the Nairobi River sub-basins, Kenya

Abstract

Effects of land-use changes on sediment yields and water quality within the sub-basins of the Nairobi River were examined. The approach used assessed spatial variations in river runoff, sediment yields and water quality and involved collection of both water and sediment samples along the river courses. Most attention was given to suspended sediments within the Ngong, Nairobi and Mathare River sub-basins. The results indicated strong seasonal trends for both suspended sediment flux and water quality status. Suspended sediments loads for the Ngong River were 1733 t km-2 year-1, Nairobi River 6317 t km-2 year-1, and Mathare River to the tune of 2987 t km-2 year-1. Close relationships were found between total dissolved solids (TDS), conductivity, turbidity and colour. Generally, pollution levels varied with season and declined with distance downstream of Nairobi due to dilution effects and self-purification of the river waters during the wet season. Strategies to control increasing sediment yields and hence water quality degradation are suggested with a focus given to the Best Management Practices (BMPs) within the watershed and the country in general.