Maridalsvannet is a lake in Maridalen, Oslo, Norway. It is the largest lake in the municipality of Oslo, serving as its primary drinking water supply. The lake drains via the river Akerselva to the Oslofjord. The primary inflows are Skjærsjøelva and Dausjøelva. The Hammeren Hydroelectric Power Station exploits the fall from Skjærsjøen to Maridalsvannet.Wikipedia