Environmental impact assessment of Large Dams became mandatory in Iran in 1996, while Shahid Rejaei dam, built in the province of Mazandaran, was under operation at that time. Therefore, there was no report available about the environmental impacts of Shahid Rejaei dam. The main goal of this study was to investigate adverse effects of the constructed dam. Then, a mitigation plan was recommended. To achieve this goal, some information was gathered about physical, biological, and socio-economic characteristics of the upstream basin of the dam. In this paper, a list of activities, performed during the construction of the dam, was introduced in details. Then, the Matrix method was applied to quantify the effects of such activities on the environment. Finally, based on the information and the results obtained, an analysis was performed for different possible variants. Based on the analysis, some recommendations were proposed to mitigate the environmental impacts of the Shahid Rejaei dam.