Mjøsa is Norway's largest lake, as well as one of the deepest lakes in Norway and in Europe. It is the fourth-deepest lake in Norway. 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 1931–1982) is 316 cubic metres per second (11,200 cu ft/s) which is about 9,959,000,000 cubic metres per year (1.1145×1010 cu ft/Ms). Mjøsa contains about 56.2 km3 (13.5 cu mi) of water compared to the 14.8 km3 (3.6 cu mi) in the lake Røssvatnet, the second largest lake by volume in Norway.

The Gudbrandsdalslågen is a river which flows through the Gudbrandsdal valley in Innlandet county, Norway. The 204-kilometre (127 mi) long river runs through a large valley in Eastern Norway before emptying into Mjøsa, the largest lake in Norway. The river flows through the municipalities of Lesja, Dovre, Sel, Nord-Fron, Sør-Fron, Ringebu, Øyer, and Lillehammer.