Reservoir sedimentation diminishes water storage and coastal delta resiliency.

Surface water storage in dammed reservoirs diminishes over time due to sediment accumulation. Here, we applied the RATTES model to predict storage loss and sedimentation at 57,117 reservoirs across the continental United States (1700-2050), resulting in cumulative reservoir sediment volumes of 63.8 km3 by 2025 and 83.0 km3 by 2050. Although cumulative storage loss is 7.7%, over two-fifths of reservoirs have lost more than 25% of designed storage capacity. The equivalent sediment mass is 61.3 Gt, exceeding the annual global fluvial sediment load by a factor of seven. If coastal deltas received the annual sediment load currently stored in reservoirs, delta aggradation rates would be comparable to eustatic sea-level rise at half of US deltas. While 79% of reservoir sediment is stored over 500 km from the coast, 7% is sequestered behind terminal dams where sustainable sediment management could reconnect sediment transport to coasts.