Assessment of Hydraulic Suitability for Chinese Sturgeon Spawning Downstream of Gezhouba Dam Under Different Flow Allocation Scenarios

Dam regulation alters riverine hydraulic conditions, potentially limiting fish spawning habitat. This study evaluated how discharge allocation could improve hydraulic suitability for Chinese sturgeon (Acipenser sinensis) spawning downstream of Gezhouba Dam. A three-dimensional hydrodynamic model, evaluated against historical near-bed velocities under approximate boundary conditions, simulated nine steady-state scenarios combining total discharges of 7090, 10,500, and 15,100 m3/s with Dajiang-to-Erjiang flow allocation ratios of 3:2, approximately 2:1, and 4:1. Hydraulically suitable areas were quantified using near-bed velocity and water-depth thresholds, and operational feasibility was discussed. Total discharge primarily controlled overall near-bed velocities, whereas flow allocation altered local velocity distributions. Integrated hydraulically suitable areas were largest under an approximately 2:1 allocation at low discharge and 4:1 at medium and high discharges, exceeding those under conventional 3:2 allocation by 34.1%, 23.5%, and 8.4%, respectively. These scenarios generally remained relatively favorable under joint threshold perturbations. Historical mean water-temperature records indicate a shift in the estimated thermal suitability window toward November–December. In a warming climate, these results identify candidate allocations for ecological operation within thermal suitability windows, subject to engineering constraints. Together with the 2025 side-channel spawning observations, they could inform small-scale spawning trials under controlled hydraulic conditions to complement mainstem regulation amid engineering disturbances.