Development of rural areas near rivers without considering flood risks can lead to increased danger and financial loss. In a study of three branches of the Halilrood River in Kerman Province, flood zones were mapped using HEC-RAS in a GIS environment. By analyzing flood levels for different return periods, it was found that over 90% of the area could be affected by a 200-year flood. Increasing the roughness coefficient doubled the flood area, showing a direct link between return period and flood extent. This research confirms the accuracy of using GIS and HEC-RAS for flood zoning and river modeling.