Impacts of human activities on the riverbed morphological in the tidal reaches of the Yangtze River

Reservoirs and river management projects have operational impacts on the runoff, sediments, and morphologies of tidal reaches. Therefore, it is of great significance to analyze the influence of human activities on the hydrology and geomorphology of the tidal reaches of the Yangtze River (YR). We measured topographic and water-sediment flux over the past 40 and 60 years, respectively, to determine changes in the characteristics of riverbed lowering or deposition, shoal areas, and diversion ratio of branch channel (DRBC), along with the dynamics and driving mechanisms. The results indicated that cumulative scouring of the tidal reaches occurred from 1981 to 2021 (time-averaged annual riverbed lowering depth = 4.35 cm), which was mainly controlled by reductions in sediment influxes, followed by sand mining and waterway dredging. Riverbed lowering or deposition in the tidal reaches were characterized by deep channel lowering and shoal deposition from 1981 to 2001. From 2001 to 2021, lowering of both deep channel and shoal occurred, with the riverbed lowering volume of the low-flow channel accounting for 79.3 % of the total riverbed lowering volume. The shoal areas exhibited an increasing trend from 1981 to 2001, but reduced by 2.35 % in the 2001-2021 period mainly due to the operation of Three Gorges Reservoir (TGR). Moreover, the implementation of waterway improvement projects slightly increased the shoal areas since 2016. Particularly, the DRBC accounted for 15-50 % of the increase in bifurcation ratio during the 1962-1981 periods. Reductions in sediment influxes and unbalanced riverbed lowering caused by adjustments of flow processes increased the DRBC from 1981 until prior to the implementation of the river and waterway improvement projects. After the implementation, further increases in the DRBC were effectively inhibited or the ratio was decreased. In the near future, the amounts of sediment influxes in the YR Basin will remain low or even further decrease, leading to sustained or intensified scouring of the tidal reaches, and the impacts of shrinking shoal and scouring of branch channels should be focused upon. Overall, the findings of this study provide an in-depth understanding of the impacts of human activities on the riverbed morphologies of tidal reaches.