Lake Malaren is situated in a region with Archaean rocks. The deposits around the lake, especially its northeastern and southern parts, are dominated by lime-rich clays which make these areas well suited for cultivation. Central Sweden is a highly cultivated area where the use of inorganic fertilizers contributes to the nutrient loading on the lakes.

Lake Malaren is a very complicated system of waters composed of bays of different character. On the basis of its topography the lake can be divided into five basins each with its own chemical and biological status.

An early description of the lake mentions its importance as a route of communication between Stockholm, the capital, and many inland towns, the traveller being able to enjoy the many beautiful bays en route (Fischerstrom, 1785). Another comment from this early period, more interesting from a water quality point of view, says that the water usually has a good taste and is sound enough to drink. Water-blooming algae, Aphanizomenon flos-aquae ,were however observed in shallow creeks.

The quotient between drainage area and lake surface is about 20 which indicates a large surrounding area contributing to the loading on the lake. The expansion of population and industry around the lake has been and is still very high. Lake Malaren has become more eutrophic during the last 30 to 40 years. Its importance as a freshwater reservoir is steadily increasing.

Stockholm, the capital of Sweden, is situated at the outlet of Lake Malaren into the Baltic. Seventy-five percent of the inflow to Lake Malaren enters the western part of the lake but the inflow into the northeastern area is also important. This means that the water flows mainly in two directions, one from west to east and another from northeast to south before entering the connection with the Baltic.