Gravel-Sand Transition of the Yangtze River: Human Disturbances, Migration Processes, and Controlling Factors

Despite abundant studies on gravel-sand transitions (GSTs), GST origin and migration in large alluvial channels characterized by significant variations in width, numerous tributary (or distributary) streams, and frequent human disturbances remain difficult to explain. Here, we take the Yangtze River as an example and use field observations and numerical modeling to make progress toward addressing GST origin and migration. The results show that the Yangtze River exhibits a non-abrupt GST approximately 60 km long. Within the GST and the adjacent area, there are two dramatic decreases in shear stress; the upstream one induces the settlement of sand from suspension, explaining the emergence of the Yangtze River GST, whereas the downstream one induces general cessation of gravel movement. However, the deposited gravel was mostly overwhelmed by sand. We argue that these changes in shear stress in the Yangtze River depended primarily on the combined effects of width variability and distributary systems. Since 1970, the Jingjiang Meander Cut-off, the Gezhouba Dam, and the Three Gorges Dam (TGD) have been completed in succession, repeatedly altering the flow-sediment regime within the GST. However, the GST position remained unchanged until the TGD began operation in 2003. The stability of the GST was primarily ascribed to the sufficient upstream sand supply (grain size <0.5 mm), the resistant lithologies upstream of Zhicheng, and the Jingjiang Great Levees constraints.