Flow Processes in a Diffluence‐Confluence Unit on a Large‐Shallow River Under Different Discharges

Abstract Understanding the hydrodynamics of bifurcation and confluence zones is essential for effective river management. This study investigated the complex flow structure in a large diffluence‐confluence unit (width‐depth ratio >100) of the Middle Yangtze River, utilizing systematic field observations of three‐dimensional velocities under low to high discharges (8,560–46,100 m 3 /s). In terms of streamwise flow, increasing discharges caused the bifurcation node to migrate downstream, and the flow diversion ratio of secondary branch rose as the incoming discharge increased. As for secondary current, it exhibited notable changes at bifurcation zone across different flow conditions. No significant, channel‐scale secondary flow cells were was observed at low discharge, and a single secondary cell formed at medium discharge; then dual face‐to‐face secondary cells developed at bifurcation zone at high discharge, with the surface flow directed inward and near‐bed flow moving outward. To summarize, the variation in flow patterns was collectively driven by streamline curvature, local geomorphological features, and topographic steering. Flow curvature generated centrifugal forces, altering the rotation direction of secondary flow along the reach. Abrupt changes in wetted cross‐sectional area caused flow separation, intensifying both downward and transverse motions and thereby enhancing secondary flow. Furthermore, the main flow preferentially entered the primary branch with lower bed elevation at low discharge; as the inflow discharge increased, the effect of topographic steering weakened, producing a more uniform velocity distribution and consequently raising the flow diversion ratio in the secondary channel. This provides a transferable mechanistic framework to understand flow dynamics in anabranching rivers globally.