Long-term changes in lake and river systems in Finland
Finland possesses 72 waterway systems of over 200 square kilometres in size. The five largest of them account for the majority of the country’s surface area. Furthermore, there are almost 188,000 lakes of more than five ares in size. Altogether 9.9 percent of the surface area consists of water. The lakes have
been created over the last 10,000 years or so, either through the emergence of their basins from beneath the ice sheet of the last glaciation or, in most cases, through isolation from the Baltic basin. In this article, the post-glacial history of the main watercourses and lake basins in Finland and the effects of recent human activity on them and on the quality of their water are examined.
On account of the uneven pattern of land uplift, most of the large waterway systems, such as the Saimaa, Päijänne, Näsijärvi, and Puula watercourses of the Lake District, have altered their outflow in the course of time, some of them more than once. The majority of these hydrological changes took place in the interval 8500–4500 BP. In the case of Lake Oulujärvi and Lake Vanajavesi, a transgression that has been going on for thousands of years has led to rises in water level of as much as 10–15 metres. In the case of Lake Höytiäinen, uncontrolled erosion of the outflow channel in connection with an effort at lowering the water level led to a sudden drop of almost ten metres. Elsewhere, the changes in outflow channels meant that the main watershed in the Lake District shifted up to 300 kilometres in a SE–NW direction. Meanwhile, human activity has led to the total or partial drainage of about 3,000 lakes in different parts of the country. About four-fifths of the inland water area in Finland is of good or excellent water quality and only five percent is of moderate or poor quality.