Lake Mjosa, the largest lake of Norway, is a typical Norwegian fjord lake. It has a surface area of 365 km2, maximum depth of 449 m, mean depth of 153 m and maximum length of 117 km. Large parts of the catchment area consist of mountainous regions with gabbroic and granitic bedrock. Around the central zone of the lake there are sedimentary and metamorphic rocks of cambro- silurian age and some calcareous rock types. This district is one of Norway's best agricultural areas. The lake lies well below the highest post-glacial marine limit which in this area is just 200 m above the present ocean level. In the mountain regions there are many glaciers, and the main tributary, River Gudbrandsdalslagen, carries a heavy silt load from the glacier area during the summer. Approximately 75% of the water mass in Mjosa flow through the River Gudbrandsdalslagen (mean annual flow 250 m3 sec-1) which drains a region dominated by high mountains and glaciers. This tributary has been regulated for hydro-electric power generation and the mean winter water flow of the river has thus increased from 64 to 104 m3 sec-1 (65%), while the mean summer flow has decreased from 509 to 451 m3 sec-1 (11%).

Melting of snow and ice in the mountains causes a high flow during summer which is especially heavy during periods of warm and rainy weather. Approximately 60% of the annual water flow takes place between June and August. Lake Mjosa is used as a reservoir for hydro-electric power generation. The regulation height is 3.61 m which is equivalent to a reservoir approximately 2.3% of the lake volume. Water is drawn from the lake during winter (November-April) and the lake is refilled during the spring flood in May/June. The vernal circulation period normally lasts from the latter half of April to the end of June, while the autumnal circulation down to approximately 200 m lasts from October to January/February. Cooling of the deep water evidently continues until the vernal circulation period begins. The deep water temperature always ranges between 3.5 and 3.7deg C. The central zones of the lake are often ice-free during the winter.

During the period from the beginning of the 1950's to the midst of the 1970's the lake has developed strongly in an eutrophic direction. However, satisfactory solutions are to be found for waste treatment in rural districts by the "Save Mjosa Campaign" including the rapid development of purification techniques and building of sewage pipelines (1).