The downstream reach of the Bahmanshir River is sensitive to water-quality changes due to tidal influence, reduced freshwater inflow, agricultural withdrawals, drainage return flows, and its connection with saline downstream waters. This study analyzes the effects of three main scenarios, including baseline conditions, post-dam construction, and full operation of the downstream dam and navigation lock, on dissolved oxygen, biochemical oxygen demand, and the relative self-purification capacity of the Bahmanshir River using HEC‑RAS. Geometric, hydraulic, water-quality, and meteorological data were adopted from the technical reference report and field monitoring data, and the analysis was focused on DO and BOD. The results showed that, under baseline conditions, mean DO and BOD were 7.8 and 4.2 mg/L, respectively. After dam construction, DO decreased to 7.6 mg/L and BOD decreased to 3.9 mg/L. Under full operation of the dam and navigation lock, DO increased to 8.1 mg/L. However, because the changes in DO were small and comparable to the model error, they should not be interpreted as conclusive evidence of significant deterioration or improvement in water quality. The findings mainly indicate the relative response of the model to operational changes and highlight the need for simultaneous monitoring of DO, BOD, temperature, salinity, flow discharge, and residence time in water-quality management of the downstream Bahmanshir River.