The purpose of this study is to survey the thermal regime and eutrophication states in Ilam reservoir in Iran as the case study. For this purpose and to find solutions for improving the water's quality in the reservoir, two general strategies for reducing the entering pollution loads and water depletions from the reservoir's outlets were analyzed by use of the CE-QUAL-W2 model. Results of the simulation of the present situation show the existence of thermal stratification during summer, which results in the qualitative stratification in the reservoir. According to the qualitative criteria, the Ilam reservoir's state is between mesotrophic and eutrophic. Results of the scenarios of reduction of the nutrients show that in the scenario of 50% reduction of the phosphorus and nitrogen loads into the reservoir, the state of the reservoir would recover from eutrophic to semi-eutrophic. Also, release of water from the reservoir during September, October and November would cause the restoration of the quality of water in the reservoir. To avoid the occurrence of critical eutrophication in the reservoir, reducing the ponding time in the reservoir by fast depletion, preventing entrance of the upstream villages' sewage and agricultural drained waters, which are sources of nitrate and phosphate contamination into the rivers, and also management of the usage of agricultural fertilizers have been suggested.