This amounts to one quarter of the total lake surface in Poland. There are about 60 artificial water reservoirs whose size exceeds 100 ha. The largest of them is Wloclawek Reservoir which covers more than 7,000 ha. Most of the lakes in Poland are shallow. The depth of over 20 meters is very rarely encountered.
Lakes are a transitional and short-term phenomena.
All of them are slowly waning as a result of silting-up, overgrowth, erosion processes and reclamation. Depending on the initial lake depth and the rate of aforementioned processes, the waning of lakes proceeds at different speeds. The water accumulated in the lakes has a moderating influence on the environment and through increased evaporation affects the amount of precipitation. Many Polish lakes perform the role of receptors of industrial and residential effluent, which worsens the water quality. Some lakes are also polluted because of artificial fertilizers washing off of surrounding farmland.