Lake Kortowskie was studied during water stagnation, when it is subjected to bottom water withdrawal to outflow through a pipeline. The examinations were carried out after flow reduction from 0.25 m3· s-1 to about 0.05 m3· s-1. It has been revealed that hypolimnion withdrawal has much more effect on the temperature and oxygen content in the water body. Destruction of the thermal settings and the rate of oxygen depletion oc-curs with variable intensity, depending on the amount of water withdrawn by the pipeline. In the period of limited hypolimnion withdrawal the temperature increase near the bottom was slower (by 0.2°C on average in 10 days) than in the period of the pipeline’s maximal capacity (i.e. 0.5-0.8°C).