This study aimed to identify the hydrologic characteristics and investigate the effect of hydrological flow changes for the Adhaim tributary by updating the information of watershed hydrology and examined the possibility of changing the spillway dam from being ungated to a controlled gated spillway to reduce the impact of water releases on the Tigris River and Baghdad City during the flood season. In addition, an attempt is made to store as much water as possible to benefit the summer growing season by supplying water to irrigation projects downstream of Adhaim Dam. The results indicated that if the gates were installed at the level of 135 m above sea level, flood waves with a probability of 1/100 can be controlled and safely flow to the Tigris River at ~200 m3/s. The gates would also increase the capacity of the Adhaim reservoir. Different options for controlling the gated spillway are suggested, and the designers can choose one of them.