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.
