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 km north of Oslo. Its main tributary is Gudbrandsdalslågen in the north; the only distributary is Vorma in the south. Inflows would theoretically need 5.6 years to fill the lake. With an average depth of 153 meters, most of the lake's volume is under sea level. Average outflow (measured 1931-1982) is 316 m3/s or 9959 million m3 per year. Mjøsa contains about 56 km3 of water compared to 15 km3 of Røssvatnet, the second largest volume of lakes in Norway.