A forecast of the hydrochemical state of the water in the reservoir of the Kankunskaya hydroelectric plant is provided. The expected contents of organic and biogenic substances, dissolved oxygen and volatile phenols are calculated. The study revealed an increase (compared to the background) in contents of organic and biogenic substances and volatile phenols in the range of allowable concentrations, and a decrease in the amount of dissolved oxygen. It was found that the river runoff is the main source of these substances. The submerged woody vegetation is not a substantial contributor to the deterioration of water quality.Keywords: hydroelectric engineering, river, impounded river bed, extractable substances.