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As influenced by human activities, including high dam construction, soil and water conservation, and sand mining in the upper reaches of rivers, the amount of sediment entering the lower reaches of the river decreases significantly. Human activities disturb the natural balance between river evolution and water as well as sediment movement and induce new characteristics of river evolution. After the construction and operation of the Danjiangkou Reservoir and the Three Gorges Reservoir of the Yangtze River (China), different degrees of bend bypassing and shoal cutting in the bends downstream of the dams are observed. The reason for this interrelation has not been explained sufficiently. Based on the analysis of prototype observation data and physical model experiments, we have investigated the evolution mechanism of different topographic types of meandering rivers under the condition of decreasing sediment inflow. The data generated by our model experiment document that the sediment concentration of the convex bank flow with a large degree of curvature of the bend is lower than that of the concave bank flow for a reduction of the sediment concentration from 100% to 0%. Moreover, the convex bank of the bend gradually collapses and retreats, whereas the main channel of the river gradually swings, with a swing range of 0–800 m, toward the convex bank. This phenomenon becomes more significant with progressive reduction of the sediment inflow. Combined with theoretical analysis, we interpret that the drastic reduction in the amount of sand that has destroyed the relative natural balance of sediment transport in the riverbanks and river channels in the meandering river section is the main reason for the occurrence of bypassing and shoal cutting in the meandering river section of plain alluvial rivers. Especially during the mid-to high-water period, the capacity of sediment transport in the mainstream belt exceeds the sediment concentration in the water body significantly and is coupled with the change of the runoff process caused by the regulation of the reservoir operation. As a consequence, the river beach is constantly scouring and developing. The evolution is generally characterized by bend bypassing and shoal cutting. The results of our study can be transposed to other alluvial plain rivers and may constitute valuable reference data.

Dam construction often changes downstream fluvial processes by reducing sediment supply. Taking Tiaoguan reach and Laijiapu reach of the lower Jingjiang Reach downstream of the Three Gorges Project as examples, three-dimensional flow velocity, sediment, and bed elevation were observed in the two bends for investigating the impact of flow structure and sediment transport of different discharges on sharp bend morphology. Results indicated that the flow structure and sediment transport process in curved channels depended upon the flow stages, which affected the patterns of erosion and deposition along the point bars and concave banks. Flow separation and development of secondary flow were depended on the shapes of point bars and flow depths nearby, and the strength of secondary flow increased with flow discharge. The high flow discharges, which had high sediment carrying capacity and stream power, provided the main driving force for erosion on upstream point bar, thus the type and duration of floods were crucial factors in the morphological evolution of meandering bends. The reduction of sediment supply should be responsible for erosion on the point bars, causing the flow to migrate toward the convex banks. In meandering rivers with reduced sediment supply, retreats (push inward) of inner (convex) banks dominated advances (pull inward) of outer (concave) banks. In addition, the formation and development of concave-bank bars might relate particularly to meander curvature. This study is expected to constitute a reference for bank protection and river management in meandering bends downstream of reservoirs.

After the construction of the Three Gorges Dam (TGD) in China, the downstream has been affected by the reduction in sediment discharge and regulation of flow processes, which have resulted in severe scouring and changes hydrological regime. Consequently, the spawning ground of Chinese sturgeon distributed along the downstream Yichang reach could be affected. This study examined the effects of TGD on the streamflow, sediment load and channel morphology downstream based on in situ measured data. Results showed that, after the impoundment of the TGD, sediment load at the downstream Yichang hydrological station decreased significantly, and the Yichang reach continued to be scoured. The distribution of erosion was uneven, and the scouring mainly occurred in the branching channels. The channel gradient and riverbed roughness increased with the erosion of the river cross section. After more than 10 years of erosion, the riverbed scouring and armouring in the Yichang reach was basically completed, thus we expected that the spawning grounds of Chinese sturgeon could be retain as the riverbed tends to be stable. The findings in this work have implications in the protection of the critically endangered Chinese sturgeon.