Using multi-proxy analysis to determine the long-term impacts of catchment dynamics on water reservoirs - A case from a tropical reservoir (Ruiru Basin Kenya).

Globally, siltation of water reservoirs is a major risk and cost to the provision of fresh water. Therefore, managing reservoir sedimentation is a significant task for water management agencies. In Kenya, the Ruiru water reservoir, one of four water reservoirs supplying Nairobi with drinking water has experienced a significant loss of volume since construction in 1949. However, there have been no studies characterizing the long-term catchment dynamics such as erosion, sedimentation and accumulation in the reservoir. A detailed understanding of the sediment dynamics such as identification of sediment source area; processes driving soil erosion in the catchment and accumulation in the reservoir, and identification of flood layers are necessary for the reservoir and landscape management. The accumulated sediment has not been characterized; therefore, long-term land-use and climate change impacts on the catchment on the reservoir are not documented. The study aims to identify the historical land use and climate events in the catchment impacting the reservoir through a multiproxy sediment characterization of the sediment accumulating in the Ruiru reservoir. An undisturbed 1-m sediment core retrieved in 2017 from Ruiru Reservoir was dated using Pb and Cs and particle size distribution and chemical element profiles analyzed. The accumulated sediment is a predominantly fine-grained red silt, with a particle size end-member analysis identifying four possible sources or processes of sediment accumulation. The multi-proxy analysis reveals six periods of significant accumulation, 1949, 1963/64, 1974/75, 1982/83, 1997/98 and 2013/14. The peaks coincide with high rainfall events and two are attributed to significant land-use changes in the catchment. The study identifies the catchment dynamics with a significant sediment input into the reservoir. This highlights the importance of reservoirs as environmental archives documenting 20th century land -use and climate events while providing a long-term perspective for management of critical water infrastructure.