Towards an ecologically based regulation practice in Finnish hydroelectric lakes

Water level regulation due to hydropower production has caused notable changes in the littoral zones of regulated lakes and rivers. Heavy geomorphological changes have taken place in lakes with a raised water level, and a lowering of the ice cover during the winter causes rapid changes in the littoral benthos and vegetation. The scale of the harmful effects depends on both the range of regulated water level fluctuations and the water quality; clear water lakes are more resistant to water level regulation than humic lakes. The regulated lakes in northern Finland were subjected to intensive ecological research during the 1980s. Most of the studies were conducted by the water authorities and power companies. As a result of these studies, the principles of so‐called ecologically based regulation practices (ERP) have been applied to several lakes under hydropower production. This procedure is based on under water light climate and water level fluctuation data, which make it possible to calculate the proportion of the frozen littoral to the total littoral area. Another procedure calculates the biomass of the benthic fauna from data on water level fluctuation and Secchi depth. The ERP offers a simple way to illustrate to the regulation‐permit owners differences between various regulation practices. In the first case study on Lake Kostonjärvi, the new regulation practice was developed in 1991. The new practice included a higher water level target during the winter and a well‐defined target for the spring flood water level. During the open water period, high water levels should be avoided. The ERP was successfully put into effect with the exception of 1994, although the annual mean loss for hydropower production was US $0.14 million. In the second case study on Lake Oulujärvi, interest was focused on the higher summertime water level, which should prevent the spread of emergent vegetation. Owing to the high costs of the ERP (US $5 million) a new plan with a lower summertime water level target (122.5 m) and other complementary rehabilitation methods, including removal of shore vegetation, was applied in Oulujärvi.