The Songzi River is one of the primary channels through which the Jingjiang River diverts water into Dongting Lake, and changes in its water diversion have significant implications for flood control in the river-lake system, water resource utilization, and water environmental ecology. Based on analysis of measured data and physical model experiments, this study investigates the relationships among water-sediment evolution, riverbed scouring, and diversion changes in the Yangtze main channel and Songzi Mouth reach following the operation of the Three Gorges Project.The results show that compared with the period from 1990 to 2002, the annual average sediment transport at Zhicheng Station decreased by approximately 90.6% during 2003–2022, with a synchronous reduction of about 87.1% in sediment diversion at Songzi Mouth. From 2003 to 2022, the bankfull channel between Zhicheng and Yangjiaonao experienced a total scouring volume of 239 million m3, with an average scouring depth of 2.3 m, leading to significant riverbed coarsening. In contrast, the bankfull channel at the Songzi Mouth reach was scoured by 43.25 million m3, with an average scouring depth of 3.0 m, forming a differential scouring rate compared to the pebble-sand mixed bed in the main channel.Physical model predictions indicate that long-term action of subsaturated flow will cause continuous downcutting of the sandy riverbed downstream of Yangjiaonao in the Yangtze main channel, leading to progressive declines in water levels at Yangjiaonao and corresponding reductions in diversion at Songzi Mouth. For a discharge of 7000 m3/s at Zhicheng Station, water levels at Yangjiaonao are projected to decrease by 1.58 m and 2.57 m in 2035 and 2050, respectively, resulting in diversion reductions of 8.3% and 16.7%. However, asymmetric scouring between the inner and outer parts of the mouth (with higher scouring rates in the sandy mouth reach, expanding the cross-sectional flow area) leads to increased diversion at Songzi Mouth by the end of 2035, even considering the water level decline at Yangjiaonao. During low-flow periods (with a discharge of 7000 m3/s at Zhicheng Station), diversion actually increases by 20.8%, with contribution rates of -33% from the Yangjiaonao water level drop and 133% from asymmetric mouth scouring. As the discharge increases, the absolute values of both contribution rates rise significantly, but their combined effect results in minimal net change in diversion volume, suggesting that these influences are primarily concentrated during medium- to low-flow periods.The differential riverbed composition regulates the scouring process and maintains stable diversion at Songzi Mouth. Increased low-flow diversion is beneficial for water resource allocation in Dongting Lake, but it may moderately lower main channel water levels and affect navigation during medium- to low-flow periods.