Episodic reservoir flooding transforming sediment sinks to sources and the potential global implications

Reservoirs are widely recognized as sediment sinks, but the role of silted-up reservoirs as sediment sources remains poorly understood. This study presents quantitative assessments on episodic erosion effects of reservoirs in the Yellow River. Here we showed that sediment erosion during the 2019 flooding was 112 and 265 million tons at the Sanmenxia and Xiaolangdi reservoirs, respectively. Nearly all the Sanmenxia erosion was attributable to natural floods, whereas only ~40% of the Xiaolangdi erosion resulted from natural floods with the rest from drawdown and artificial flooding. These erosions contributed ~1/4 of decadal fluvial sediment flux. Reservoir erosion increased downstream suspended sediment concentration by over two orders of magnitude. Threshold siltation ratios (sedimentation volume/storage capacity) for initial erosion in the studied reservoirs ranged from 28% to 87%, averaging 60% ± 21%. Considering many reservoirs experiencing >30% capacity loss and intensifying floods under global warming, reservoir erosion will become more frequent worldwide.