Downstream geomorphic impact of the Three Gorges Dam: With special reference to the channel bars in the Middle Yangtze River

The Three Gorges Dam (TGD) has altered downstream flow–sediment regimes and led to significant changes in the morphodynamic processes in the Middle Yangtze River (MYR). However, due to the complexity of this large river, the driving forces and implication of the morphodynamic processes remain insufficiently understood. This study selected two typical meandering and bar-braided reaches, the Zhicheng (ZC) and Shashi (SS) reach, to examine their responses to the TGD operation. The results showed that in the post-dam period significant channel erosion occurred with a higher erosion rate in the ZC reach (closer to the TGD) compared with the SS reach. The area of the Guanzhou mid-channel bar (ZC reach) and the Sanba mid-channel bar (SS reach) shrank by 30 and 90% from 2003 to 2015, respectively. The increased fluvial erosion intensity due to the reduction in suspended sediment concentration (SSC) drove the shrinkage of the mid-channel bars, as demonstrated by empirical relationships between bar geometry and fluvial erosion intensity. An increase of 22 days per year in the frequency of post-dam medium-to-high discharges (10 000–25 000 m³ s⁻¹), and associated with the reduction in SSC, jointly led to the greater erosion at the convex (inner) banks than the concave (outer) banks, which has negatively affected the designed navigation channels at the concave banks by decreasing their discharge partitioning ratios. The post-dam water level at a given high discharge (>25 000 m³ s⁻¹) showed no evident change, but the water level at a given low discharge (<10 000 m³ s⁻¹) decreased. The reduction in water levels at low flows can affect water supply and riverine ecosystems in the MYR. © 2019 John Wiley & Sons, Ltd.