Mjøsa is the largest lake in Norway and the fourth deepest in Norway and Europe. It is located in the southern part of Norway, about 100 kilometres (62 mi) north of the city of Oslo. Its main tributary is the river Gudbrandsdalslågen flowing in from the north; the only distributary is the river Vorma in the south. Inflows would theoretically need 5.6 years to fill the lake. With an average depth of about 150 metres (490 ft), most of the lake's volume is under sea level. The average outflow of the lake (measured from 1931 to 1982) is 316 cubic metres per second (11,200 cu ft/s) which is about 9,959,000,000 cubic metres per year (1.1145×10 cu ft/Ms). Mjøsa contains about 56.2 km (13.5 cu mi) of water compared to the 14.8 km (3.6 cu mi) in the lake Røssvatnet, the second largest lake by volume in Norway.
With a surface elevation of about 121 metres (397 ft), the depth of Mjøsa means that the deepest part of the basin is located approximately 332 metres (1,089 ft) below sea level. This is lower than the deepest point of the sea inlet of Kattegat and the lower than the vast majority of Skagerrak off Norway's south coast. Mjøsa retains a larger average depth than both the North Sea and Baltic Sea respectively.
