Using the HEC-HMS model to model dam reservoirs with different scenarios and different return periods allows for the prediction of inflow and outflow flows as well as an accurate assessment of the performance of these reservoirs in flood control. By using hydrological models and simulating different scenarios, the performance of these dams in flood control is examined so that the necessary solutions can be developed in water management and flood control. The Tang-Sorkh reservoir-regulating dam is one of the important dams under construction between Fars, Kohgiluyeh and Boyer-Ahmad provinces, which, in addition to providing drinking water to the cities of Shiraz and Yasuj, is also responsible for controlling seasonal floods. In this study, the effect of this dam on reducing peak discharge and flood volume in the Bashar River watershed at the entrance to Yasuj city has been investigated using the HEC-HMS 4.9 model and the HecRas-2D model. In the first stage, modeling of the basin without a dam reservoir was performed to calibrate the model. In the calibration stage, the initial model was implemented using three flood events with a 6-hour rainfall distribution at the Yasuj station. After the calibration process, the model was evaluated with another flood event in the validation stage. The average values of the Nash-Sutcliffe coefficients NS and the root mean square error RMSE were obtained as 0.87, 0.37 and 0.90, 0.30, respectively, in the calibration stage and in the validation stage. These results showed that the observational and simulated data are in good agreement with each other and the accuracy of the model in simulating the Bashar River basin is acceptable. In the second stage, the modeling of the Tange-Sorkh reservoir dam was carried out. Using the HEC-GeoRAS extension and HEC-RAS software, first the data related to the volume-height curve of the dam was prepared. Then, according to the type of dam spillway, the reservoir was routed and the dam was modeled for the return periods of 2 to 100 years in three scenarios of empty, half-full and full reservoir. The results showed that this dam can reduce the peak discharge on average in different scenarios in the half-full and full reservoir for the return periods of 2 to 100 years, respectively, compared to the situation without a reservoir, by 58.3 and 45.5 percent. Also, the flood volume will be reduced by 66 and 2.6 percent in the three scenarios for empty, half-full and full reservoirs, respectively. In addition, with the increase in the peak discharge and flood volume in different return periods, the effect of the reservoir on reducing the peak discharge and flood volume decreases.