Tinnsjå is one of the largest lakes in Norway measuring about 51.38 km2 (19.84 sq mi). It is also one of the deepest lakes in Europe, reaching a depth of 460 m (1,510 ft). Tinnsjå is located in the municipalities of Tinn and Notodden in Telemark county. At its source in the west, the Måna river flows out of the lake Møsvatn and past the town of Rjukan into Tinnsjå. From the north, the river Mår flows from the lakes Mår, Gøystavatn, and Kalhovdfjorden into Tinnsjå. Tinnsjå is part of the Skien watershed, and it drains via the Tinnelva river in the south, down to the lake Heddalsvatn.