ИСТИНА

It is usually believed that there is a high amount dissolved oxygen in rivers, as it comes to water from the atmosphere and is generated by algae during photosynthesis. However, our studies of many rivers in the European territory of Russia (ETR) have shown that river waters are undersaturated with oxygen. The reason is a large amount of organic matter in rivers which come from the river catchment. Bacteria consume dissolved oxygen during organic matter decomposition processes. Analysis of data collected by ROSHYDROMET during many years for rivers of Kuban, Terek, Don, Volga, Pechora, and Severnaya Dvina catchments (about 400 sites,1988-2011) has shown that the oxygen regime of rivers changes considerably from the South to the North. The average oxygen content decreases, its yearly amplitude and the winter deficit grows (expressed in saturation percentage). The orographic factor makes a significant contribution to the spatial distribution of oxygen in rivers: less slope provides slower flow in river and lower of saturation of water with oxygen from air. Very important is the duration of the ice cover period, varying from 7 months in the North to 0 days in the South. In the ice cover period, oxygen exchange between water and air stops, usually there is no photosynthetic aeration of water; however, bacteria continue to consume oxygen. The content of organic matter in water naturally increases from the South to the North in ETR. This is caused by increasing of specific discharge washing out organic matter from soils. It changes from 1 l/s*km2 for Don to 10 l/s*km2 for Severnaya Dvina. With an increase in the concentration of organic matter in water, its decomposition by bacteria increases and more oxygen is consumed. Thus, the geographic conditions are important for the oxygen regime of the ETR rivers.