Characteristics of runoff and sediment transport during flood events in the upper Yangtze River, China, in the last 50 years

Understanding changes in sediment loads during flood events is crucial for elucidating river sediment transport, particularly in the upper Yangtze River Basin, China, where several flood events contribute the majority of the annual sediment load. This study analyzed data collected from selected hydrological stations from 1976 to 2020 by utilizing statistical analysis, hysteresis analysis, and mathematical modeling to examine changes in runoff and sediment load during flood events in the upper reaches of the Yangtze River. The study's results indicate that the annual runoff changed little, but the annual sediment load has generally decreased, which is closely related to soil and water conservation efforts and dam construction. The operation of reservoirs disrupts the continuity of flood and sediment transport along rivers. For floods, reservoirs change floods propagation, transforming them from dynamic waves in the river channels to kinetic waves in reservoir area. And for sediment, on the one hand, there is an abrupt change in sediment transport downstream, and the large amount of sediment intercepted by reservoirs is the main reason for this abrupt change. On the other hand, the change in hysteresis characteristics observed during flood events exhibits a sudden transition. Before 2003, the Jinsha River (main sand yield area in the upper region) downstream to the Three Gorges Reservoir experienced a gradual shift in sediment peak pattern, from hysteresis characteristics predominantly reflecting a leading sediment peak to predominantly reflecting synchronized sediment and waterflow peaks. Due to reservoir operation, the hysteresis characteristics of the control area change to predominantly reflect a lagging sediment peak, for which changes in the mode and speed of propagation of the flood and sediment peaks are the main causes. The findings of this research are highly valuable for sediment resource management and reservoir regulation in the upper Yangtze River Basin.