The catchment basin covers a total area of 5,372 square kilometres (2,074 sq mi).

[citation needed] The lake is part of the Skien watershed and is connected to the ocean by the Telemark Canal.

Heddalsvatnet is only 16 metres (52 ft) above sea level and only two locks at Skien were needed to allow ships to sail on the lake.

The canal opened in 1861 and made Notodden into Norway's largest fresh water port.

[7][8] The water in the lake gradually became freshwater around the Bronze Age, according to archaeologist Pål Nymoen.