Oxygen regime improvement in large, lowland reservoirs during the winter period for reduction of fish suffocation: the example of the Dnieper reservoirs, Ukraine

Two methods of improving oxygen levels in series of large, lowland reservoirs in order to reduce fish suffocation during prolonged cold winter conditions are presented. The tool for this improvement is the management of water release through hydroelectric power stations (HEPS). The first method is the enlargement of the ice-free area downstream of the HEPS by increased water release through the dam. The second method is the short-term elevation of water level in the reservoir to increase the water mixing in deep areas where fish overwinter. The former method can be effective for riverine zones, the latter for lacustrine reservoir conditions. These two techniques are demonstrated for the Dnieper reservoirs, Ukraine. The mathematical formulas for calculation of the HEPS releases and their effect on the oxygen regimes are presented, as well as examples of calculations for the Kaniv and Kremenchuk reservoirs.