<p>Floodwater is a natural and integral part of people&#39;s life in all urban and rural societies. Therefore, providing strategies for controlling this natural phenomenon and reducing its damages is necessary. Construction of detention dams is one of the structural methods for controlling floodwaters which is studied in this article. In the present research, Neka river drainage basin was selected and HEC-HMS software was used for simulating the reservoir. In HEC-HMS software, it is necessary to define a method for routing the reservoir, determining the height and type of the outlet structure, determining reservoirs&#39; level-height equations, etc. After completing the required information and implementing the model, floodwater hydrographs were compared in both before and after situations. Results showed that HEC-HMS software can be used as a powerful tool in the geometric and hydraulic simulation of detention and storage dams in designing hydraulic projects.</p>