Closure dams at island heads are widely adopted in waterway regulation.However,the mechanism by which variations in the crest elevation of such dams affect the spatial hydrodynamic characteristics of compound bifurcated channels remains poorly understood.Based on the typical morphological features of mid-channel bars and compound channel cross-sections,this study establishes a three-dimensional turbulent numerical model to systematically simulate the flow structure of the diversion reach under three sets of closure dam crest elevations and hydraulic conditions.The results show that closure dam and mid-channel bar produce a combined backwater effect,which significantly increases backwater height at the island head.The backwater influence extends upstream by approximately 3-4 times the width of the main channel and shifts the high-velocity zone of the main branch toward the point bar.The planar velocity distribution and separation zone morphology are primarily controlled by the channel cross-sectional geometry,whereas the dam crest elevation mainly adjusts the flow field discharge,but upstream propagation of the backwater attenuates this intensity.Under high downstream water level conditions,when point bar depth exceeds main channel depth by 35%,the average velocity difference between the two reaches is more than 12%.It is inappropriate to simply simplify the compound section into a single section according to the water depth ratio.The research results can provide a theoretical basis for optimal design of closure dams and improvement of navigable flow conditions in similar reaches.