Transformation of the depocenter in the Yangtze Estuary under fluvial sediment changes and human intervention

The global decline in fluvial sediment delivery to mega-deltas has raised widespread concerns. In the Yangtze Delta, the response processes of the sediment depocenter, under the coupled influences of fluvial sediment decline (>70% since the 1950s) and rapidly developing estuarine engineering projects, remain inadequately studied. To clarify this, we studied seabed sediments, bathymetry, and sediment diversions in the estuarine channels during 1982, 2012, and 2021. Our results reveal a general trend of sediment coarsening, with the median grain-size isolines consistently migrating seaward over the past forty years. However, the mechanisms driving this migration varied significantly. Temporally, early migration (1982–2012) occurred under conditions of relatively high fluvial sediment supply and overall deposition, reflecting the combined effects of delta progradation and subsequent hydrodynamic reworking during the transitional reduction in sediment input, while recent migration (2013–2021) reflects the erosional transformation of the abandoned depocenter margin under sediment starvation and engineering disruption. Spatially, migration rates of the isolines varied between the North and South Channels, shaped by differences and dynamic changes in sediment diversions ratios. This study underscores the vital impacts of fluvial sediment changes and human engineering on sediment dynamics within the Yangtze Estuary, providing critical insights that can inform sustainable estuarine management and sediment conservation strategies.