A systematic review of the hydrogeomorphological impacts of large dams in Africa

Abstract

Large dams exert significant impacts on the hydrology and geomorphology of the rivers they impound. Although there is a renewed interest in large dams in Africa to support sustainable development in the face of a changing climate, no systematic review of their hydrogeomorphological impacts exists at continental scale. In this review, we compiled a geospatial dataset of 1047 large dams from different data sources (jointly storing up to 948.7 km³; equal to 29 % of the continent's average annual discharge), and systematically investigated the impact these dams have on water and sediment regimes, river and coastal geomorphology. Our findings reveal a consistent augmentation in low flows and a pronounced reduction in high flows (resulting in an average 35 % reduction in the coefficient of variation of monthly discharges), and often, a decrease in average river flows. The total sediment retention by these dams was estimated at 459.6 Mt. yr^-1, with fluxes towards the Mediterranean Sea being particularly reduced by 197.6 Mt. yr^-1. Although trends in altered flow regimes and trapping efficiencies (average of 85.5 %) display 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. This research significantly advances our understanding of the water infrastructure and its potential challenges for sustainable water and sediment management in the context of a changing climate and ever-high erosion rates.