Rivers are usually exposed to floods that cause significant human and financial loss, for which structures are considered in the rivers for preventing floods and reducing damage. In this way, it is necessary to acquire design discharge for building these structures. The case study was conducted on the Tar River of Ghazi Station in Tehran Province, which originates from 1 km west of Tar Lake in 13 km east of Damavand. The Tar River is one of the tributaries of Damavand River. In this study, the cross section of the river was first measured through surveying and existing maps. The design discharge of flood control structures was designed using the methods described in this study. Moreover, the stage-discharge table was used to obtain the Manning roughness coefficient. The effective discharge was calculated using the sediment rating curve and sediment frequency histogram, while the bankfull discharge was derived from the return period of 17 years discharge record and fitting of Gumbel distribution to the data. The results indicated that the average of dominant, effective, and bankfull discharges could be the appropriate design discharge for the river, as their values are significantly close to each other. However, there is no need to use flood control structures in this river, due to the occurrence of medium frequent flood events in the river.