This research article published by MDPI, 2021
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 catchmentwide 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.