LAUSR

River flooding—the world’s most significant natural hazard—is likely to increase under anthropogenic climate change. Most large rivers have been regulated by damming, but the extent to which these impoundments can mitigate extreme flooding remains uncertain. Here the catastrophic 2016 flood on the Changjiang River is first analyzed to assess the effects of both the Changjiang’s reservoir cascade and the Three Gorges Dam (TGD), the world’s largest hydraulic engineering project on downstream flood discharge and water levels. We show that the Changjiang’s reservoir cascade impounded over 30.0 × 10 m/s of flow at the peak of the flood on 25 July 2016, preventing the occurrence of what would otherwise have been the second largest flood ever recorded in the reach downstream of the TGD. Half of this flood water storage was retained by the TGD alone, meaning that impoundment by the TGD reduced peak water levels at the Datong hydrometric station (on 25 July) by 1.47 m, compared to pre-TGD conditions. However, downstream morphological changes, in particular, extensive erosion of the natural floodplain, offset this reduction in water level by 0.22 m, so that the full beneficial impact of floodwater retention by the TGD was not fully realized. Our results highlight how morphological adjustments downstream of large dams may inhibit their full potential to mitigate extreme flood risk. Plain Language Summary The role of damming in modulating flood risk in large rivers and the issue of how flood risk changes is attracting considerable interest. We develop the first study to address how the cascade of reservoirs behaves under extreme flood conditions in terms of the interplay between the positive effects of impoundment versus the impacts of downstream morphological changes. Our findings are highly significant to those dealing with the theory (hydrology, physics, and geosciences), application (hydrology, geomorphology, ecology, biology, and geochemistry) and implications (social science, policy, and environmental management) of flood risk in the world’s large rivers in the context of rapid climate warming and extensive dam regulations.