Sediment retention by large dams in Africa

Despite their crucial role in water management and hydropower generation, African dams are often overlooked in global dam research. This study examines the geographic distribution and characteristics of large dams in Africa, resulting in a newly compiled database of 1047 large dams with a collective storage volume of 948.7 km³, representing 29% of the continent’s annual discharge. Considering the critical impact of sediment retention on downstream rivers and coastal systems, we estimated the total sediment retention by these large dams. To do this, we applied Brune’s (1953) widely used relationship between trapping efficiency (TE) and the ratio of a reservoir’s storage capacity (C) to its average annual water inflow (I). Storage capacity data were sourced from our database, while a 1 km-gridded runoff dataset provided the average annual water inflow. We then linked the calculated trapping efficiencies with sediment yield data for Africa, and we accounted for the sediment cascade and interdependencies between dam catchments. For 616 dams, representing 98% of the total storage volume, sufficient data allowed us to estimate total sediment retention at 459.9 Megaton per year (Mt yr⁻¹). Significant reductions in land-to-sea sediment fluxes were observed for the Mediterranean Sea (197.6 Mt yr⁻¹), Indian Ocean (74.5 Mt yr⁻¹) and Gulf of Guinea (56.6 Mt yr⁻¹), with additional reductions to the North Atlantic Ocean (42.0 Mt yr⁻¹), South Atlantic Ocean (27.6 Mt yr⁻¹), and within endorheic basins (61.5 Mt yr⁻¹). Our estimates are consistent with reported data at catchment level, and comparable to sediment retention in major river basins such as the Yangtze and Pearl Basins, though slightly lower. Our findings highlight the increasing importance of catchment management and restoration. As 40% of electricity in Africa south of the Sahara is generated from hydropower and irrigation water supply becomes increasingly important, mitigating storage capacity losses is essential, especially in light of climate change intensifying the hydrological cycle, leading to higher evaporation losses and higher sediment yields.