Simulating floods and creating vulnerability maps are essential in urban watershed management systems. The present research aimed to simulate floods and assess floodplain risk in the Khalkai watershed located in Gilan Province, Iran. This river is characterized by extensive agricultural areas and urban areas that become flooded during most rainy seasons. Hence, investigating this topic is important. In this research, a two-dimensional hydrodynamic modeling approach was used to simulate floods with probability return periods of 50, 100, and 500 years, under the influence of GPS topographic mapping compared with DEMs extracted from satellite images. Therefore, MIKE21 and HEC-RAS 6.3 software were utilized to construct flood inundation maps and select the best model for the Khalkai watershed. The following objectives were considered in this study: 1. The impact of the roughness coefficient index and curve number was examined due to increasing simulation accuracy, so the modeling was carried out for both steady and flood flows. By examining the rate of change in the roughness coefficient index and the curve number, it was shown that these two factors vary depending on the type of flow state in the simulation. The main reasons for this difference are the flow velocity, infiltration, and flow regime in the area, which are under critical conditions due to human interventions such as bridge and wall construction, which increase the probability of significant damage. 2. The accuracy of floodplain estimation was evaluated using the abovementioned models with Sentinel satellite images taken during a 25-year flood event. The accuracy of the models used in estimating the floodplain density was subsequently assessed.