How does hydropower station construction reshape the geomorphology of downstream mid-channel bars?

Abstract

Fluvial geomorphological changes have profound implications for the health and functionality of river ecosystems. The influence of hydropower dams on downstream geomorphology and the environment demands immediate investigation. However, current research methods are intricate, posing substantial challenges. This study focuses on the mid-channel bar (MCB), a crucial fluvial landform, to explore how hydropower development affects downstream fluvial geomorphology. Using global surface water datasets, the study analyzed nearly 20 years of MCB data from downstream of 10 hydropower stations along the Yangtze, Yellow, and Pearl Rivers. The results indicated that: 1) From 2000 to 2021, the total area of MCBs downstream of hydropower stations in major Chinese rivers experienced a remarkable decline. Approximately 80 % of the reduction (4.83 km²) occurred in the Yangtze River. The rate of decrease varied among river basins. The Yangtze River experienced the steepest decline at a rate of −0.23 km²/yr, followed by the Yellow River (−0.05 km²/yr) and the Pearl River (−0.01 km²/yr). 2) The impact of hydropower damming on changes in MCB area differed before and after river closure. In most cases, the rate of decrease accelerated significantly after river closure. 3) For individual MCBs, around two-thirds showed a downward trend in area, with the rate of decline accelerating after river closure. The reduction in MCB area is primarily attributed to three factors: decreased downstream sediment supply due to sediment trapping in reservoirs, changes in flow dynamics caused by dam regulation, and internal factors such as MCB type and evolution trajectory. This study provides a novel perspective on understanding the ecological impacts of hydropower and offers scientific support for fluvial geomorphology management and ecological conservation.