Abstract: With Zhenjiang-Yangzhou reach in the downstream Yangtze River as study area, the morphological responses and trends of central bar channels to varying water and sediment discharge are investigated based on bathymetric data. Results indicate that: (1) In the presence of low incoming sediment, the branching channels of Shiye sanbar and Hechang sandbar experienced overall erosion and unidirectional deepening especially after 2010, while the middle bars are relatively stable. (2) The variations of water diversion ratios suggested that the Shiye sandbar is dominated by main channel development and secondary channel shrinkage, while Hechang sandbar is on the contrary. Recent regulation projects have effectively retrained the unidirectional change of branching pattern. (3) Channel erosion and deepening would continue under low sediment discharge in the future, leading to high risk of local bank collapse. Systematic monitoring and forecasting of bank failures are advised, particularly for several unprotected weak banks.

Key words: varying water and sediment discharge, central bar, morphologic evolution, Shiye sandbar, Hechang sandbar, the lower Yangtze River