Abstract: Over the past two decades, mid-channel bars in the branching sections of the Middle Yangtze River have experienced substantial scouring or even retreating, yet the specific variances in scouring between slightly meandering and straight channels remain unclear. Baishazhou Sandbar and Tianxingzhou Sandbar, located in the Wuhan reach of the Middle Yangtze River, were selected as study sites to investigate the causes of morphological changes and differential scouring. This investigation utilized a combination of multi-source remote sensing imagery, hydrological data, and field surveys. The findings reveal significant alterations in the morphology of Baishazhou Sandbar, shrinking in size by approximately 43.83% between 1989 and 2020. Meanwhile, the head of Tianxingzhou sandbar underwent notable changes, although the overall exposed area remained relatively stable. Analysis of the normalized vegetation index (NDVI) indicates generally low vegetation cover on both sandbars, with diminished spatial discrepancies. During dry season, variations in exposed area primarily correlate with the bar head region. Specifically, on the Baishazhou Sandbar which is located in straight channel, scouring was observed on the head and lateral side, while local collapse on the left side, both due to the obstruction of Baishazhou Bridge piers. Similarly, on Tianxingzhou Sandbar which is located in slightly meandering channel, the bar head has experienced scouring following the impoundment of the Three Gorges Project. However, the implementation of protection measures, including artificial rock rip-rap in 2004, effectively mitigated scouring and facilitated deposition. In conclusion, differential scour of mid-channel bars manifests in several aspects, including channel planform, the relative positioning of mainstream and branches, and the presence or absence of protective infrastructure.

Key words: mid-channel bar, slightly meandering reach, straight reach, differential scour, water-sediment change, Middle Yangtze River, Wuhan Reach