Lake Paijanne is the second largest lake in Finland and the major lake of the River Kymijoki watercourse in central Finland. The basin of the lake lies in the old Precambrian primary rock consisting mainly of granite and schist. Loose ground consists mostly of till. The lake was formed after the last glacial period some 8,000 years ago, when the land began to rise with the retreat of ice and the connection with the sea was severed.

Lake Paijanne is 120 km long and its width varies from 1 to 28 km. There are several separate basins along the lake. Nutrient loadings mainly enter the northern part of the lake. The water quality is therefore poorer in the northern than in the southern part.

The total area of the drainage basin of Lake Paijanne is 25 times as large as the lake itself. Most of the basin is covered by forests, the proportion of farmland being only about 10%, and forested peatlands account for 25%. From the peatlands, humic substances are discharged to the lake. The forests consist mainly of pine and spruce. The population in the drainage basin is about 400,000, of which 150,000 live near the lake. There are some 30,000 holiday cottages along the shore of the lake.

Lake Paijanne is an important water resource which supplies drinking water to about 1 million people in the Helsinki metropolitan area. Water is conducted from the lake through a 120 km tunnel. The water quality in northern parts of the lake deteriorated definitely in the 1950's and 1960's. As the result of recent legislative and administrative measures, waste loadings have now diminished substantially and water quality has been steadily improved (Q).