The results of water quality monitoring of the Saratov reservoir for the period 2000-2016 are presented. Observations were carried out monthly at a stationary station located in the lower tail of the Zhigulevskaya HPP. It is established that the quality of water in the source of water supply is characterized by signifi cant seasonal variability. During the mass development of algae the organic contamination of the reservoir increases and the concentration of nutrients decreases. Excessive content of organic substances in the summer low water complicates the technological processes of water purifi cation. It is concluded that phosphate is the limiting factor restraining the mass development of algae, the concentration of which in the peak of “fl owering” of water is reduced to almost zero.