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 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 , most of the lake's volume is under sea level. The average outflow of the lake (measured from 1931–1982) is which is about . Mjøsa contains about of water compared to the in the lake Røssvatnet, the second largest lake by volume in Norway. With a surface elevation of about , the depth of Mjøsa means that the deepest part of the basin is located approximately 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. Thomas Robert Malthus traveled through Norway in 1799 and his diaries from the trip includes a description of Mjøsa. Malthus wrote that Mjøsa appears as both lake and river because the shores are defined by mountains and where the valley becomes wider the water fills the space. Below Minde (Minnesund) the lake only appears like a river and is called Vorma on the map, according to Malthus. Location From its southernmost point at Minnesund in Eidsvoll to its northernmost point in Lillehammer it is long. At its widest, near the town of Hamar, it is wide. It is in area and its volume is estimated at ; normally its surface is about above sea level, and its greatest depth is . Its total coastline is estimated at , of which 30% is built up. Dams built on the distributary of Vorma in 1858, 1911, 1947, and 1965 raised the level by approximately in total. During the 19th and 20th centuries, there have been 20 floods recorded that added at least to the level of the lake which caused flooding. Several of these floods inundated the city of Hamar. The towns of Hamar, Gjøvik, and Lillehammer were founded along the shores of the lake. Before the construction of railways past the lake,