Influence of the water discharge of the Kuibyshev reservoir on the dynamics of the ecosystem of the Seredysh and Bakhilovsky islands, the Volga River, Russia

Abstract

The state of the ecosystem of the protected islands Seredysh and Bakhilovsky (former Shalyga) is significantly affected by the water consumption in the power site of the Zhigulevskaya Dam. The 62-year dataset was analyzed to study the patterns of temporal variability of water discharge from the filling of the Kuibyshev Reservoir to the present. Long-term variability of the water discharge is due to climatic conditions in the Volga River basin, it follows to some extent the 22-year cycle of solar activity. Natural seasonal variability of the water discharge is altered by regulation at the Zhigulevskaya Dam, especially under unfavorable conditions for the formation of the water flow. The periods of low-water, high-water and medium water content were defined in regard to the annual water consumption. The low-water period lasted 19 years from 1958 to 1976, the high water period, 20 years from 1977 to 1998, and the period of medium water availability, 23 years from 1999 through 2019. Extreme high-water and low-water years, which had an adverse effect on the functioning of the islands’ ecosystem, were calculated using the empirical data on the water flow rate. During the spring flood of extremely high-water years, the water level in the reservoir increased sharply, so a part of the island was flooded. In the summer dry season of extremely dry years, the water level decreased, so the island area increased due to the shore retreat and appearing of sandbanks around the island. The negative impact of the water discharge on the ecological state of the islands may increase if the rules for regulating the water flow at the Zhigulevskaya Dam are violated.