Investigation of sediment source and delivery dynamics in an east African hydropower reservoir using sediment tracing technology

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). The study also aimed to evaluate the soil carbon as a proxy for erosion risk in the catchment. 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. 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 outputs revealed that the Kikuletwa River with 60.3%, was the dominant contributing tributary to the total reservoir sediment and the Ruvu River 39.7%. However, downcore unmixing results indicated that the latest increases in sedimentation is moistly 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. The “soil slake test” method for soil aggregate stability in water (WSA) indicated a significant decrease in soil aggregate stability in cultivated land in comparison to other land use types which indicates that the unsustainable land use changes can thus potentially increase the susceptibility of soils to erosion by water when soil organic matter (SOM) is reduced. This study has 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 to limit soil erosion and reduce further impact to rivers and reservoirs to maintain and enhance food, water and energy security in Eastern Africa.