Inland waters

Finland has a great number of lakes and rivers as well as smaller water bodies such as ponds, streams and rivulets. The total area of all inland waters is 3.4 million hectares, which corresponds to 8% of the total area of the country. The share of species for which inland waters are a primary habitat is 6% of the total number of well-known species. However, there is a large amount of species which are so poorly known that they have been left outside this assessment. Especially many algae are included into this group. The species composition of inland waters overlaps with the Baltic Sea because of the low salinity of the sea.

Inland waters are important habitats for fresh water fish, mayflies and trichopterans. In addition, several molluscs, birds, mosses as well as vascular plants inhabit inland waters. Most of the species occur in lakes and ponds, but some running water types and small water bodies (for example springs and rivulets) are also species-rich. Inland waters are sometimes quite small in area, which makes them especially sensitive for changes in the environment and the catchment area because of a relatively high proportion of shallow water and shore.

What and where?

The definition of inland waters is simple in principle, since they include all aquatic environments above the sea level. In cases of some species it may however be problematic to distinguish between aquatic and terrestrial habitats. For example many birds and mayflies breed or live in shallow shores, and deciding whether they should be included in aquatic or shore species can be difficult.

The share of inland waters of total area is greatest in the regions of Savo, Häme and North Karelia, where the largest water bodies are found. These include the lakes Saimaa, Päijänne and Pielinen. A lot of aquatic habitats are also found in Lapland in the lake Inarijärvi area, and in Kainuu around the lake Oulujärvi. Inland water area is the smallest in coastal areas and in Lapland, excluding Inari. Rivers are however abundant in coastal area, and their significance for the landscape and biodiversity is great, even though they are small in area.

The water bodies in Finland are relatively small in volume though large in area. The lakes are shallow, their average depth is only seven meters and the lakes Päijänne, Saimaa, Pääjärvi and Inarijärvi are the only ones reaching the depth of 100 meters. The length of river and stream channels has been estimated to be 20 000 km altogether. Most of them empty into the Baltic Sea.