Abstract: Since the operation of Three Gorges Project (TGP), significant channel erosion has occurred in the middle Yangtze River (MYR) mainstream, resulting in continual adjustments in low-water channel morphology in its different sub-reaches. This paper determines the low-water channel dimensions for different cross-sections of this reach, and calculates the reach-scale low-water channel dimensions for its four sub-reaches of 2003 to 2016 using a reach-averaged method. We find that the channel was developing into a narrower and deeper geometry and its width-depth ratio was generally decreased, thereby resulting in an improvement on navigation conditions. In addition, we develop empirical relationships of the low-water channel dimensions versus the previous years’ flow and sediment regime for different sub-reaches. The relationships show that in this period, the determination coefficients were maximized in the Yizhi and Jingjiang reaches at a flow-sediment responding lag of five years, while maximized in the Chenghan and Hanhu reaches at the lag of seven years. In all these sub-reaches, the adjustments of low-water channel morphology were significantly affected by the incoming flow and sediment conditions in the previous five or seven years respectively. Overall, the low-water channel (except for the width) responded well to the altered flow-sediment regime, and its adjustment will be larger in the sub-reaches closer to the dam.

Key words: low-water river channel, flow and sediment regime, navigation condition, middle Yangtze River, Three Gorges Project