This study investigated the influence of the Three Gorges Dam (TGD) on the evolution of nearby channels in the Yangtze River (Changjiang River) system in order to better understand the environmental impact of large‐scale reservoir operations. From 2003 to 2017, the amounts of runoff and sediment transport in the Yangtze River were reduced by 3.3–14.5% and 67.8–92.7%, respectively, relative to 1955–2002 before the TGD was operational. Topographic measurements of the middle reaches (Yichang to Hukou) of the Yangtze River were analyzed from 1975 to 2017, during which time the cumulative erosion of the flood channel was 22.78 × 108 m3, and the dry channel accounted for 90.3% of the erosion. Following commissioning of the TGD, the scouring intensity of the sandy gravel section near the dam initially increased then decreased, whereas the scouring intensity of sandy sections away from the dam continued to increase. Beaches on convex banks of curved sections were scoured, and deep channels on concave banks became silted. In braided sections, the braids tended to shrink, and the diversion ratio of the main branch during dry seasons reduced, resulting in frequent branch alternation. Compared to changes in the downstream river channels of other large reservoirs worldwide, scouring from the TGD is extensive. The findings of this study are significant for river channel regulation and waterway planning in the Yangtze River and worldwide.

Intensive human activities in river basins have substantial effects on fluvial hydrological and morphological processes and developmental trends. In this study, we assessed the evolutionary characteristics and developmental trends of the fluvial morphological system of the Three Gorges Dam (TGD) downstream to understand the impacts of human activities on the fluvial hydrological–morphological system of the region. The results indicated that the Three Gorges Project (TGP) and other upstream cascade reservoirs promoted sediment retention, which in turn aggravated the trend of declining sediment load and cumulative scouring over a long distance in the channels downstream of the TGD. Approximately 84% of scouring occurred in the low‐water channels, along with thalweg scouring and riverbank collapse. Notably, extended operation of the TGP and other upstream cascade reservoirs has resulted in gradual changes in the downstream riverbeds. Specifically, we found that the most intensive scouring zone shifted downstream from the near‐dam Yichang–Chenglingji reach (0–408 km) to the Chenglingji–Hankou reach. The scouring–silting distribution in swales has also changed: the scouring of deep swales and silting on floodplains prior to the TGP have been replaced by the scouring of both deep swales and floodplains. The scouring intensity of riverbeds in the Yichang–Hukou reach (954 km) was slightly higher than that forecasted prior to the TGP; the scouring intensities increased significantly during 2013–2021, which could be attributed to the TGP and other upstream cascade reservoirs; river sand mining and waterway dredging are secondary factors. We conclude that, owing to the effects of sediment retention by cascade reservoirs in the upper reaches of the Yangtze River (or Changjiang River), the cumulative scouring intensities of riverbeds in the middle and lower reaches of the Yangtze River will continue to increase in future decades.