The temperature of the water reaches 21 to 23 °C (70 to 73 °F) in the summer months, making for refreshing swimming. It freezes in the winter and the ice crust can be as much as 2 metres (6 ft 7 in) thick. The temperature also varies with location and depth. In the village of Topli Dol south of the lake, there is a water factory "Vlasinka", producing high-quality mineral water called "Vlasinska Rosa", a renowned brand in Serbia.
There are some 70 natural water springs in the lake area.
Source: Wikipedia: https://en.wikipedia.org/wiki/Vlasina_Lake
Vlasina Lake, Serbia