Mechanisms Controlling Water-Level Variations in the Middle Yangtze River Following the Operation of the Three Gorges Dam

Understanding the mechanisms controlling downstream water-level variations after the operation of the Three Gorges Dam is important for riverine flood and drought management. However, our quantitative understanding of the multiple controls of river morphology, vegetation, and floodplain resistance on water levels in the Middle Yangtze River (MYR) remains limited. Here, we analyze changes in river channels and floodplain resistance in the MYR using 450 cross-sectional profiles as well as data on discharge, water levels, sediment, and satellite images from 2003 to 2015. Results show an overall decline in low-flow water-levels (at a given small discharge) due to severe incisions of low-flow channels caused by a sharp reduction of similar to 90% in sediment loads from 1950-2002 to 2003-2020. In contrast, high-flow water-levels (at a given large discharge) display minor changes. Our analysis shows that the notably increased floodplain resistance due to vegetation growth is likely the dominant factor elevating flood water-levels, followed by riverbed coarsening and greater fluctuations in the river longitudinal profiles. Our findings further the understanding of downstream geomorphic response to dam operation and their impacts on water levels and have important implications for riverine flood management in dammed river systems.

Automated summary

Understanding the mechanisms controlling downstream water-level variations after the operation of the Three Gorges Dam is important for riverine flood and drought management. This study analyzed changes in river channels and floodplain resistance in the Middle Yangtze River and found that low-flow water levels have declined due to severe incisions of low-flow channels caused by a reduction in sediment loads. In contrast, high-flow water levels have not changed significantly. The increased floodplain resistance due to vegetation growth is likely the main factor elevating flood water levels.