Landward shifts of the maximum accretion zone in the tidal reach of the Changjiang estuary following construction of the Three Gorges Dam

Impacts from anthropogenic activities have substantially modified the geomorphology of most of the world's large rivers. While many studies have focused on the fluvially-dominated and estuarine/delta segments of these rivers, their tidal reaches that links the river to the estuarine delta is much less extensively documented. Yet, morphological variations in these key transition zones, however, directly affect the transfer of water and sediment to the sea, and have a significant influence on the delta environment. Here, we analyze the morphological variation of the Datong-Xuliujing Reach (DXR), the tidal reach of the Changjiang River, following the closure and operation of the Three Gorges Dam, using a unique dataset combining surveys in 1992, 2002, 2008 and 2013. The results demonstrate that the DXR exhibits three different morphological development phases. When sediment supply is high (at 3.18 × 10⁸ t/y), the DXR experienced deposition (1992–2002) with the maximum accretion zone located in the middle portions of the reach. Thereafter (2002–2008) the channel underwent a major period of erosion coincident with a substantial decline of fluvial sediment supply (to 1.72 × 10⁸ t/y). More recently (i.e., during 2008–2013), the entire reach experienced deposition, but with the maximum accretion zone shifting around 100 km landward (compared to its position in 1992–2002), while the riverine sediment supply was further reduced to 1.30 × 10⁸ t/y. Our analytical modelling further reveals that a damped high fluvial discharge, induced by Three Gorges Dam regulation, and a relatively strong water level fluctuation induced by tidal forcing in the wet season, are responsible for the upstream shift in the maximum accretion zone. In addition, local variations caused by sand mining and dredging generate spatial nonuniformity in the observed patterns of erosion and deposition along the DXR. Such knowledge is of vital importance for the sustainable management of large alluvial rivers and their tidal reaches as they respond to natural and anthropogenic effects.