Abstract
This study aimed to reconstruct the sedimentation rates over time and identify the changing sources of sediment in a major hydropower reservoir in Tanzania, the Nyumba ya Mungu (NYM). Fallout 210Pb measurements were used to estimate age of sediment deposits and broad changes in sedimentation rates were reconstructed. Sedimentation peaks were cross referenced to geochemical profiles of allogenic and autogenic elemental constituents of the sediment column to confirm a causal link. Finally, geochemical fingerprinting of the sediment cores and potential sources were compared using a Bayesian mixing model (MixSIAR) to attribute the dominant riverine and land use sources to the reservoir together with changes through recent decades. Reservoir sedimentation generally increased from 0.1 g cm−2 yr−1 in the lower sediment column to 1.7 g cm−2 yr−1 in the most recent deposits. These results correlated to changes in allogenic and autogenic tracers. The model output pointed to one of two major tributaries, the Kikuletwa River with 60.3%, as the dominant source of sediment to the entire reservoir, while the other tributary, Ruvu River, contributed approximately 39.7%. However, downcore unmixing results indicated that the latest increases in sedimentation seem to be mainly driven by an increased contribution from the Ruvu River. Cultivated land (CU) was shown to be the main land use source of riverine sediment, accounting for 38.4% and 44.6% in Kikuletwa and Ruvu rivers respectively. This study explicitly demonstrated that the integration of sediment tracing and dating tools can be used for quantifying the dominant source of sediment infilling in East African hydropower reservoirs. The results underscore the necessity for catchment-wide management plans that target the reduction of both hillslope erosion reduction and the sediment connectivity from hillslope source areas to rivers and reservoirs, which will help to maintain and enhance food, water and energy security in Eastern Africa.
Highlights
IntroductionHydropower reservoirs support both water and energy security, providing essential services required for development in East Africa [1,2,3]
Integration of the changing proportional contribution and reconstructed mass accumulation rates (MARs) seems to indicate that the most recent increase in sedimentation is mostly driven by increased sediment delivery from the RV system. This finding seems to indicate that increased erosion and sediment transport from the RV system are driven by increased land use change in the catchment
This study demonstrated the potential use of geochemical profiles as a valuable alternative tool for independent confirmation of changing sedimentation rates in the context of disturbance in East African hydropower catchments
Summary
Hydropower reservoirs support both water and energy security, providing essential services required for development in East Africa [1,2,3]. They are essential for the production of climate neutral energy [4] that guarantees the long-term energy security for economic development in developing countries. Reservoirs in East Africa provide a source of irrigation and drinking water sources for agriculture purposes and aquatic habits that are beneficial for fisheries and recreation [4,5]. Hydropower reservoirs, are increasingly threatened by changing hydrological and sediment dynamics in their wider
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