Hydrophysical Factors Controlling the Formation of Hypoxic Zones in the Pregolya River

A quasi-constant hydrological front is found to form in the Pregolya River, flowing through Kaliningrad and Kaliningrad oblast, in the convergence zones of the slightly salt waters of the Vistula Lagoon and fresh river waters under the conditions circulation caused by positive and negative setups. Its formation and position are found to coincide with zones of hypoxia, especially in the places (hollows) within the city boundaries where the river depth is relatively large. A thick layer of liquid and semiliquid silts rich in organic carbon, especially in hollows; the dynamics of mobile exchange layer in bottom sediments at high temperature in summer; and the formation of the hydrological front with characteristic hydrophysical features along with an increase in the input of pollutants with municipal wastewater is a cause and a source of secondary pollution and deterioration of the environmental situation in the Pregolya River.