In Sistan and Baluchestan Province, the rivers are prone to flooding, but few studies have assessed the floods in these rivers. Given the substantial importance of the border rivers in this province, flood hydraulics for four border rivers, namely Mashkil, Surin, Gezo, and Biraban, were studied in this research. To this end, hydraulic modeling of floods and preparation of flood zone maps with 2- and 50-yr return periods were performed using the HEC-RAS model, ArcGIS software, and HEC GeoRAS extension. According to the calculation and comparison of various hydraulic parameters, in 2-year return period discharge or dominant discharge, the Mashkil river had the highest depth and extent of floods, and the Gozo river had the highest average flood velocity among the studied rivers. In the 50-year return period discharge, Mashkil and Gozo rivers had a greater potential for flooding. Finally, comparing the ratio of changes in different hydraulic parameters with the change of flood return period from 2 to 50 years indicated that the increase of discharge (5.8-28.3 times) in the studied rivers had greater effects on the flood volume (3.3-7.8 times) and its extent (2.3-4.5 times)
