Sediment and morphological changes along Yangtze River's 500 km between Datong and Xuliujing before and after Three Gorges Dam commissioning.

The impoundment of the Three Gorges Dam on the Yangtze River begins in 2003 and a full pool level is first attained in 2010. This process leads to reciprocal adjustments in flow discharge, sediment transport and morphology downstream of the dam. Based on 26-year recorded hydrologic data 1990-2015 and surveyed bathymetries 1998, 2010 and 2015, this study elucidates, before and after the commissioning of the dam, the alterations along the 500-km reach of the river. Two-dimensional numerical simulations are performed to predict future morphological changes by 2025. The analyses demonstrate that the impoundment modulates the seasonal flow discharges and traps an appreciable amount of sediment, resulting in enhanced erosion potential and coarsening of sediment. On a multi-year basis, the maximum discharge varies by a factor of 1.3 and the corresponding suspended load concentration and transport rate differ by a factor of 3.0 and 3.8, respectively. Combinations of surveyed and simulated bathymetries reveal its morphological responses to the changes. A general pattern of erosion is observed along the reach. In its upper 120 km, the process slows down towards 2025. In the middle 200 km, the erosion shifts, following the gradual impounding, to slight deposition, which then shifts back to erosion around September 2018. In the final 180 km, erosion continues without any sign of de-escalation, which is presumedly ascribed to tidal actions. The reach has not yet achieved a hydro-morphological equilibrium; the riverbed down-cutting is supposed to continue for a while. The combination of the field and numerical investigations provides, with the elapse of time, insight into the morpho-dynamics in the 500 km river reach.