Climate change and human activities in river basins have jointly altered the natural course of river channel evolution. In particular, large reservoirs have profoundly changed the water and sediment fluxes and river evolution processes and trends downstream. Considering the Three Gorges Project as an example, this study examined the impact of the operation of the Three Gorges Dam on water and sediment fluxes downstream as well as on river evolution. The findings can improve the understanding of the influence of water conservancy projects on river evolution downstream. In this study, variations in river scouring and siltation in the middle reaches of the Yangtze River (Yichang-Hukou) from 1975 to 2017 were studied. The results showed that the riverbeds in the middle reaches of the Yangtze River have undergone cumulative scouring in the past 40 years. The scouring intensity in riverbeds composed of sand and pebbles near the dam first increased and then weakened, whereas that for sandy riverbeds increased. This was mainly due to the combined effects of local hydrological and sediment transport conditions, the adjustment of the river and lake relationships in the sections, and human activities. In general, different evolutionary trends were noted for different river types and can be summarized as follows. Scour was dominant at the alluvial islands of the branching stream sections. The shapes of the islands have been stabilized with the help of the channel improvement project in the upper Jingjiang section, but the island areas have decreased owing to the discharge of clean water. The branching ratios in the dry season have decreased, and some of the main branching channels have changed into minor streams, and vice versa. Although the areas of the alluvial islands in the lower Jingjiang section and the downstream section have fluctuated, the channel improvement project is generally effective at controlling the sizes of the islands. For curved sections, scouring was dominant on the convex sides. On the concave sides of the sharply curved sections, siltation occurred in deep channels. For moderately curved sections, beach scouring was noted on the convex sides, whereas there were stable deep channels on the concave sides.