Abstract: At the entrance reach of multi-branched channels, the temporal and spatial distribution of sediment erosion or deposition often changes due to the position of main stream shifting with different income discharge. In order to investigate the relation between the magnitude of bed deformation and flow discharge, numerical simulations of flow fields are carried out and channel morphology comparisons were made across different discharge in several braided reaches of the middle Yangtze. It is found that flow dynamics exhibits a critical character when incoming discharge changes. That indicates the existence of two critical discharges Q₁ and Q₂. The head of sandbar will be eroded when the incoming discharge ranging between Q₁ and Q₂. The head of sandbar will be deposited when the incoming discharge is above Q₁ or below Q₂. Because the impacts of incoming discharge on river bed deformation are different according to different discharge magnitudes, the seasonal or annual adjustment of bed morphology at the head of sandbars will mainly depend on the duration of discharge process.