Adjustment patterns of cross-sectional morphology in the middle reaches of the Yangtze River after the impoundment of the Three Gorges Reservoir

The construction of large reservoirs warrants adjustments to the channel morphology downstream of the dam. Existing studies provide a relatively systematic understanding of the overall characteristics of channel adjustment but lack quantitative criteria for distinguishing the adjustment patterns of different cross-sections downstream of the dam. This study investigates the morphological adjustment patterns to the cross-sections of the middle Yangtze River following the impoundment of the Three Gorges Reservoir and establishes identification criteria based on fixed-section topographic data. Analysis is conducted from the perspective of the cross-sectional centroid. The main conclusions are as follows: ① In the temporal dimension, the longitudinal coordinates ( H_y） of the cross-sectional centroid generally decreased. The transverse coordinates （H_x） of the centroid in straight river types varied slightly, whereas those in meandering river types shifted toward the convex bank. Meanwhile, in bifurcated river types, H_x shifted toward the main channel. In the spatial dimension, the greatest adjustment to the cross-sectional morphology in straight-river types occurred in the lower reaches of the Lower Jingjiang River and the Jiepai River section. H_y decreased the least in the gravel-sand river section near the dam, whereas H_y changed the most significantly in the Upper Jingjiang River section. For both meandering and bifurcated river types, the greatest relative change in centroid coordinates occurred in the Jingjiang River section and gradually decreased with increasing distance from the dam. ② Based on the intrinsic relationship between shape center migration and the oscillation of the hydrodynamic axis, it has been clarified from a dynamic perspective that changes in the shape center can reflect the characteristics of cross-sectional adjustment. ③ By introducing the relative amplitude of the centroid coordinates, three cross-sectional adjustment patterns—global downward-cutting, translational downward-cutting, and translational adjustment—along with their identification criteria are proposed. Straight-river types primarily exhibit global downward-cutting, meandering-river types display all three patterns, and bifurcated-river types primarily exhibit global downward-cutting and translational downward-cutting. Compared with previous approaches, this method quantitatively distinguishes differences among river types and demonstrates that the relative amplitude of the centroid can capture the coupled variation between lateral cross-sectional displacement and vertical scouring-deposition processes.