A reliable alteration of the state of saprobity in the oligosaprobic section is possible after a shift of 15-20°C in the water temperature, whereas in the alpha-mesosaprobic section a shift of 6-12°C is enough. These changes are mainly connected with the interrelation between diatoms and filiform green algae in the first section and with bacterial activity in the second one. After mingling of the river and sewage the minimum water temperatures rise more and the maximum ones less.