Downstream channel morphological adjustments induced by mega‐dams: A comparative study of the middle Yangtze River and the lower Yellow River

The Three Gorges Dam (TGD) and Xiaolangdi Dam have triggered substantial morphological changes along the middle Yangtze River and the lower Yellow River (LYR), respectively. Yet a comprehensive comparison of these changes is lacking, which hinders our understanding of large rivers' response to mega‐dam operation. Based on systematic data of hydrology, sediment and cross‐sectional surveys over the last two decades, we compared the water‐sediment conditions and the morphological changes of the two rivers. Results showed that sediment load decreased by about 90% in both rivers while discharge reduction was minor. Both rivers underwent degradation, widening, decreased width‐to‐depth ratio (W/H), reduced slope and bed coarsening, but with different magnitudes, rates and spatial extents. The average incision depth of thalweg was 4.2 and 4.6 m, and the maximum was 24.9 and 11.9 m at the Yichang‐Chenglingji Reach (~380 km) in the middle Yangtze River and the LYR (~730 km), respectively. The upper Jingjiang reach experienced the most severe degradation within the middle Yangtze River, with no temporal declining trend of the erosion rate. Erosion centers were primarily located in the braided reach of the LYR. The erosion rate at this reach decreased recently after about 2017, while it increased at the downstream transitional and meandering reaches. The changes in the LYR agree with Lane's balance, and W/H was negatively correlated with water discharge multiplied by slope. Heterogeneous changes along the channels prohibited the application of simplified, unified frameworks for channel adjustment to regulated water and sediment.