A systematic review of the hydrological and geomorphological impact of large dams in Africa

 

Despite the fact that large dams exert a significant environmental impact on the rivers they impound, a renewed 21^st century interest in these hydraulic structures exists, mainly driven by the premise to make the agricultural and energy sectors climate-resilient. This study focuses on the often underexposed large dams in Africa and entails an examination of their spatial distribution and characteristics, and downstream impacts. To this end, we have created a comprehensive spatial database of 1047 large dams, which can be considered the most complete database for Africa. Together, these dams impound 949 km³ of water for irrigation (50% of dams), urban and industrial water supply (31% of dams), hydropower generation (16% of dams, with a total installed capacity of 43,567 MW) and other purposes (< 3% of dams). The findings of our systematic literature review of the hydrological and geomorphological impact of these large dams reveal a consistent augmentation in low flows, a pronounced reduction in high flows, and often, a decrease in average river flows. Furthermore, sediment trapping efficiencies within the range of 60% to 99% are documented. Although these trends exhibit broad consistency across the continent, the associated geomorphological changes frequently exhibit localized variations. Common alterations encompass riverbed incision and a narrowing of the active riverbed. Coastal erosion and the permanent opening or closing of estuaries are also recurrently observed. Additionally, a spectrum of ecological impacts is identified, ranging from thermal and hypoxia pollution to shifts in riparian woody vegetation distribution, often favouring non-native species. Although there is no one-size-fits-all solution to address these environmental externalities, our study highlights the importance of environmental impact assessments, monitoring of dam-induced impacts and environmental justice. Furthermore, the study offers insights into potential forthcoming challenges in the context of a changing climate  (32% and 44% of the dams respectively expect significant precipitation increases and decreases by 2050) and ever-high sediment yields (at 73% of the dam locations, sediment yields above 10 Mg km^-2 yr^-1 occur). These challenges necessitate adaptive dam operation strategies and transboundary management, and additional efforts to deal with reservoir sedimentation.