Optimal exploitation of dams’ reservoirs and provision of dam downstream water demand is of great importance. The use of water resource planning models has been recently very effective. For this purpose, this paper has analyzed and investigated the optimal utilization of Galmandareh River basin water resources using a combination of MODSIM as a simulation model and election algorithm as an optimization model and the development of the EA-MODSIM model. The objective function of the problem was considered as minimizing the sum of squared deviations between the required amount and the amount released from the reservoir. Efficiency, reliability, and vulnerability in reservoir operation were also used to evaluate the proposed model. Results indicated the acceptable performance of the simulation-optimization approach used in the research to solve the problem of planning the optimal allocation of water resources at the basin. Studies showed that in the simulation-optimization approach, the amount of water demand in the basin increased by about 32% compared to the simulation model. The results of the reliability index for industrial, agricultural, and environmental demands at the dam downstream were 86, 87, and 96%, respectively. These results indicated the importance of adopting policies for the optimal exploitation of system reservoirs to increase the water supply and reduce the water resource loss at the level of the basin.