One of the main sources of water supply in the Ukrainian part of the Danube delta is the Danube lakes, one of which is Katlabukh. The main revenue part in the water balance of the adjacent delta Lake Katlabukh is the inflow of Danube water through canals with offtakes. The water levels in the lake are determined with the rules of the use of the reservoir, as well as with the hydrological regime of the Danube. Water exchange is carried out by off takes Hromadsky and Zhelyavsky. The reduction of water exchange processes with the Danube in combination with anthropogenic impact on the catchment area of small rivers flowing into Lake Katlabukh, as well as the negative phenomena associated with climate change; create many environmental, water, and social problems for the lake. One of the main problems is the unsatisfactory state of water quality in terms of hydrochemical parameters, in particular high mineralization, which limits the use of the lake water for water supply. The permanents monitoring and analysis of the hydrochemical state of the lake waters will allow to development of scientifically based recommendations for improving water quality and rational use.