<p>This study evaluates the dynamics and capacity of discharge and flow of the Al-Salal River below the Salal Bridge in southern Iraq to assess the performance of the 1D-HEC-RAS hydrodynamic model under steady-flow conditions. The model was driven with upstream discharge, and the downstream boundary was set as a time series of water levels due to tidal impact caused by the Arabian Gulf. It was modeled using six different operational setups with variations in the lateral opening (open and closed) and the marsh water level (0.0, 1.8 and 3.0&#xa0;m a.s.l.) and the direction of the flow (into or out of the Hammar Marsh). The discharge rates were 50–600 m³/s. The calibration and validation used field data, namely daily averages of the water levels and flow velocities during six months (temporal resolution: 1&#xa0;h) at six cross-sectional sites. The model showed strong reliability for water surface elevation with Manning’s n = 0.035 (R² = 0.86, RMSE = 0.004&#xa0;m, NSE = 0.81) and moderate accuracy for flow velocity (R² = 0.74, RMSE = 0.005&#xa0;m/s, NSE = 0.74). The open-opening rating curve (Cases 1–2) of the bridge cross-section is shown, which yields that under open bridge conditions (open bridge configurations), maximum discharges are 600 m³/s while maintaining velocities below 0.6&#xa0;m/s and water levels under 2.48&#xa0;m On the other hand, local velocity peaks up to 0.9&#xa0;m/s with the closure of lateral openings (Cases 3–6), which cause serious erosion risks to riverbanks and bridge foundations. The paper highlights the importance of structural countermeasures to increase the mitigation aspect in the high-velocity areas, including stone plugging or slab pillars. These results suggest that hydraulic infrastructure and environmental processes are interconnected and provide practical information on flood management and sustainable planning of water resources in the ecologically fragile area of southern Iraq.</p>