Using statistical methods, an analysis of the runoff long-term distribution of the Southern Bug River in its lower reach in damming and natural conditions has been performed. It allows deepening the existing knowledge about the regularities of the water ecosystem functioning and rationalizing (balancing) the water supply system in the regional socioecological system, with the purpose of prevention of ecologically dangerous situations and minimization of ecological and economic damage. At the same time, the main emphasis is shifted to the study of the South-Ukraine electric power producing complex impact on the ecological safety in the lower reach of the Southern Bug River aquatic ecosystem (after the cascade of reservoirs). In particular, it has been proved that the river runoff decreases, and this phenomenon is particularly acute in the low water flow period when there is a problem of prioritizing the allocation of water resources between industry, population and the ecological system actually. In the statistical environment R and software MS Excel, the degree of connection of water flow in the lower reach of the Southern Bug River with the concentration of potentially dangerous substances in it is determined, and, for some of them, a functional dependence is presented. An analysis of the database of observation of daily water consum ption over 80 years (from 1936 to 2016) has allowed substantiating and proving the opinion that the annual runoff of the river tends to decrease. The probability of low water flow for the months of the year for the lower reach of the Southern Bug is calculated and a probabilistic distribution of water flow is constructed. The economic risk for the South-Ukraine electric power producing complex, and to the Ukraine’s energy system, in particular, in the low water flow period, when the values of the river water flow become dangerously small for the normal functioning of the enterprise, under the terms of the set volume sanitary discharges in its lower reach is determined.