Global patterns of reservoir sedimentation and overlooked risks in small reservoirs

Reservoir sedimentation is an escalating threat to global water security. While most existing assessments focus on large, well-managed reservoirs, they overlook the vast and vulnerable network of smaller reservoirs, which hampers our understanding of global sedimentation risks. Here we present a high-resolution global assessment of reservoir sedimentation in over 550,000 reservoirs, ~95% of which are <1 km2, based on a physics-guided machine learning model and a comprehensive reservoir inventory. Sedimentation rates have been underestimated in over 75% of reservoir-bearing regions when large reservoirs alone are considered. This oversight conceals a loss in global average reservoir water storage of 7.3% ± 2.8% per decade, with nearly one in five reservoirs now facing high-risk status. We identify 16 global high-sedimentation hotspots across the Western Americas and Afro-Eurasian Sediment Belts, threatening water supplies to over 2 billion people and impacting ~26% of global irrigated land. Without intervention, we project that, by 2060, more than half of all reservoirs could become functionally inoperable, including 58.6% of small and 38.1% of large reservoirs. These findings highlight an urgent need for targeted, nature-based sediment management, particularly in dryland regions, where small reservoirs are critical for domestic water access and food production. Global reservoirs are under threat from increasing sedimentation rates, which threaten infrastructure function and storage capacity. A focus on large reservoirs underestimates the scale of the problem and the importance of resilient small reservoirs for global water security.