Changes in progradation rate of the tidal flats at the mouth of the Changjiang (Yangtze) River, China

Progradation rates of tidal flats at the Chiangjiang River mouth, China, over different periods were examined, based on surveys and historical records. Rapid progradation after the Holocene sea-level rise began about 2000 years ago because of an increase in riverine sediment. The tidal flats at the river mouth have grown at a rate of about 5 km²/year and the southern mainland shoreline in front of the river mouth has advanced at a rate of 17 m/year. In recent decades, the lateral progradation rate at the frontal area has been from tens to hundreds of meters per year. The evolutionary process of the tidal flat is highly episodic under this general tendency of progradation. Spring-neap cyclicity shows clearly in calm seasons while the bare flat surface is sensitive to windy conditions. Different seasonal patterns occur between the marsh and the bare flat because of the influence of vegetation. Spatial changes in sedimentation rate are also striking, which could be attributed to differences in sediment distribution and hydrodynamics. The changeability of sediment surface shows a shoreward reduction from the subtidal area to the high marsh, likely due to the shoreward decrease in water energy and submergence time as well as the protective effect of marsh vegetation. It is predicted that the general progradation could be greatly slowed when the sharp reduction in riverine sediment caused by the Three Gorges project and the South-To-North Water Diversion will coincide with the rapid relative sea-level rise.