Coastal impacts of damming and water abstraction in the Tana and Athi-Sabaki river basins of Kenya

Abstract

This study was carried in the Kenya’s largest river basins which contribute 90 % of the freshwater discharged into the Kenyan sector of the Indian Ocean. While there has been an increase in sediment load of the Athi-Sabaki River, the load in the dammed Tana River has been declining. The state changes that are attributed to the increase in the sediment load of the Sabaki river included siltation, estuary morphological changes (shallowing), changes in the mean flow, extension of mangrove forest, decreased water transparency, macro algal blooms and biodiversity degradation. In case of the Tana River where sediment load has declined due to damming, the state changes were coastal erosion, saltwater intrusion, degradation of biodiversity, decline in floodplain agriculture, changes in mean flow, morphological changes (estuary deepening) and increased turbidity. The impacts of water abstraction on the hydrology of the Athi-Sabaki and Tana rivers was found to be low due to the relatively low levels of exploitation for industrial, domestic, livestock and irrigation purposes. Climate variability, as opposed to water abstraction and damming was found to be responsible for the long-term variability of the flow of the Tana and Athi-Sabaki River systems. While the magnitude of river discharge determines the quantity of sediments transported by the two rivers, land use change in the catchment area, characterized by increased deforestation to open land for agriculture and settlement and the need to satisfy increased wood demand, is responsible for the generally high sediment loads of the two rivers.