Shangchewan Oxbow is a typical abandoned river channel in the middle reaches of the Yangtze River, serving critical functions in species protection, biodiversity conservation, and water supply. To investigate the evolution of its physical morphology and the corresponding response of hydrological connectivity, remote sensing interpretation was used to extract the waterbody morphology from 1990 to 2021. Physical morphology indices-including the shoreline development index, morphological development index, and contraction index-were identified and computed. A combined Lasso regression and Mann–Kendall (MK) trend analysis was then applied to characterize nearly 30 years of morphological evolution. In addition, a method for calculating the hydrological connectivity between the abandoned channel and the main stream of the Yangtze River was proposed, and the impacts of connectivity on physical morphology (including key threshold ranges) were analyzed. The results indicate that both the shoreline development index and morphological development index have increased significantly following the operation of the Three Gorges Reservoir, while the contraction index has declined. These trends reflect an ongoing evolution toward an oxbow-lake form and an increasing influence of human activities. In addition, the narrowing of the meander section and sedimentation of the downstream portion of the channel are evident. The balance thresholds for hydrological connectivity with respect to the morphological development and contraction indices are determined as [1.75, 2.15] and [2.1, 2.5], respectively. When the connectivity exceeds 2.5 or falls below 1.75, the oxbow is at risk of gradual disappearance.