Declining Sediment Discharge in the Yangtze River From 1956 to 2017: Spatial and Temporal Changes and Their Causes

A sharp decline in the Sediment load at the seaward most Datong station (DTS) of the Yangtze River occurred at an average rate of 7.22 x 10(6) t/yr since 1956, and this drop had a staircase pattern that can be divided into five distinct periods (1956-1968, 1969-1991, 1992-2002, 2003-2012, and 2013-2017). In this study, a new holistic basin-wide Sediment Budget Method (SBM) was developed using sediment data from a total of 74 gauging stations (arguably the largest datasets so far) to estimate the sediment contributions to DTS from tributaries and mainstream. Our results show that the Jinsha River in upper basin between Panzhihua and Xiangjiaba used to be one of the largest contributors to DTS in 1956-2002 (16%-33%), but its contribution decreased sharply after 2003 due to the impoundment of Three Gorges Dam (TGD) and decreased further to nearly nil in 2013-2017 due to the impoundment of new cascade dams (CDs) which were upstream of TGD. In contrast, the mainstream between Yichang and Datong contributed 50%-64% of the DTS in 2003-2017, owing to the active channel erosion downstream of TGD. Before 2003 about 66%-84% of the DTS was contributed by the upper reach, but 72%-91% of the DTS was from the middle-lower reaches after 2003, reflecting a drastic shift in sediment suppliers from upper to middle-lower reaches. On average, the drop of DTS was 128.37 x 10(6) t/yr due to TGD impoundment in 2003-2012 and 27.28 x 10(6) t/yr due to CDs impoundment in 2013-2017 because of the reservoir trapping and the inhibition of historically eroding channels in the reservoir area. This new SBM can be applied to other large and small rivers around the world to track sediment sources/sinks and provide valuable results for sediment resource management.