Abstract: To explore the mechanism of the impact of topographic changes in an anabranching channel on flood carrying capacity, this study takes the Lunzhou Reach of Beijiang River in Qingyuan as the research object. It not only analyzes the topographic change characteristics of the anabranches under the action of the "22·6" extreme flood and after dredging, but also studies the response laws of the anabranch discharge ratio, water level, flow velocity, and flow direction to topographic changes based on a two-dimensional flow mathematical model. The results of topographic changes show that there are significant branch-specific differences in the scour-silting pattern of this reach: the upper section of the right branch is dominated by silting, the middle and lower sections of the right branch have a complex scour-silting pattern, while the left branch has a relatively weak overall scoursilting intensity but the scouring intensity increases along the course. The results of model simulation indicate that: under the designed floods with frequencies of 0.33%, 1%, 2% and 5%, topographic changes cause the discharge ratio of the right branch to decrease by 0.03, and that of the left branch to increase accordingly; the degree of influence of topographic changes on the water level, flow velocity, and flow direction of the anabranches is negatively correlated with flood discharge. Specifically, the water level change is within ±0.1 m, the flow velocity change is within ±0.5 m/s, the flow direction change is within ±10°, and the changes in water level and flow velocity of the left branch are significantly weaker than those of the right branch.