Sandy riverbed shoal under anthropogenic activities: The sandy reach of the Yangtze River, China

Large-scale reservoirs change downstream water and sediment conditions, causing adaptive adjustments in erosion and deposition along the riverbed, ultimately impacting sandbar morphology. This study examined the sandy reaches downstream of the China Three Gorges Project along the Yangtze River for river erosion and siltation, riverbed morphology, beach evolution, and other physical processes over the past 40 years, to explain the evolution of river geomorphology under human activities, such as reservoirs, improvement projects, and sand mining. The Shashi reach is approximately 52 km in length; it is the first sandy riverbed downstream of the China Three Gorges Dam. Analysis of topographic data from 1975 to 2018 showed that decreased sediment from the basin resulted in increased riverbed erosion (93.1%, in low-water channel), with sand mining activities destroying the integrity of the central bar. Simultaneously, total sandbar area decreased, thalwegs deepened, river morphological coefficients decreased, and the riverbed became narrower and deeper. The erosion and deposition patterns of the Shashi reach beach and channel have changed from "low shoal erosion-high shoal deposition" to "both low and high shoal erosion". Additionally, waterway engineering has changed sandbar boundaries, and impacted the evolution of interactions among beach groups. Under quasi-natural conditions, strong interactions occur in sandbar evolution, such as the development of the Taipingkou Channel Bar (TPKCB), which led to Lalinzhou Point Bar (LLZPB) erosion, tail section widening, as well as Sanbatan (SBT) shrinkage and recession of its eastward edge. Owing to the implementation of waterway engineering, the linkage between the TPKCB and the upper section of the LLZPB weakened. The widening of the lower section of the latter has caused the SBT to shrink further and shift westward. Changes in the morphologies and dynamic axes of upstream beach groups have resulted in a sharp reduction of incoming sediment causing the downstream Jinchengzhou Point Bar (JCZPB) to gradually evolve into a shrunken channel bar.