The important role of tributaries and flood events in sediment transport: A study on the spatiotemporal variability of water and sediment discharge upstream and downstream of the Three Gorges Reservoir

After the construction of the cascade reservoirs in the lower Jinsha River, it has become necessary to clarify the spatial and temporal variability of the volume of sediment entering the Three Gorges Reservoir in order to manage sediment within the basin considering the new situation of rapid sediment reduction. Previous studies were affected by the inaccurate analyses of transition points and associated causes and were unable to identify clear changes in the spatial and temporal characteristics. Therefore, the impact that the Three Gorges Reservoir has on sediment reduction in the upper Yangtze River needs to be reassessed. The Lepage test and sediment identity factor method were applied to reveal that the sediment transported by the Jinsha River has significantly decreased since 2013 due to the reservoir construction, while the sediment transported by the Min River and Jialing River has slightly increased since 2014 and 2015 due to heavy rainfall. However, this has ultimately led to a significant reduction in sediment entering the Three Gorges Reservoir since 2013. Accordingly, the spatial composition of sediment sources in the Three Gorges region has been dominated by tributaries in recent years, with the Jialing River accounting for 38.98 % of the total and the Jinsha River accounting for only 1.74 %. Under these new conditions, sediment transported during short-term floods becomes more important, with sediment entering the Three Gorges Reservoir during 7 day flood events accounting for up to 47.16 % of the total transport in a year. As a result of sediment trapping by the upstream reservoirs, the Three Gorges Reservoir is responsible for only 7.5 % to 22.75 % of the sediment reduction observed at Yichang Station. Understanding the weakened impact that the Three Gorges Reservoir has on the sediment reduction phenomenon in the upper Yangtze River is important for managing the schedules of the reservoirs in the upper Yangtze River.