The 1951–2004 time series of annual sediment supply from the Yangtze were analyzed using the wavelet method. Coastal bathymetric data were processed using the Arc‐GIS software. A typical intertidal flat was measured from May 2002 to August 2005 for comparison of bed levels before and after the Three Gorges Dam (TGD). There is a significant decreasing trend in riverine sediment supply since late 1960s, which is attributed mainly to the dam constructions. TGD reduced the sediment load to147 mt/yr in 2004, only 35% of the average. In response to this drastic decrease, the deltaic coast is turning from progradation to recession. We predict that the Yangtze sediment supply will further decrease and coastal erosion will be intensified in the coming decades, which poses a great challenge to coastal management.