Water level regulation due to hydropower production has caused notable
changes in the littoral zones of regulated lakes and rivers. Heavy ge
omorphological changes have taken place in lakes with a raised water l
evel, and a lowering of the ice cover during the winter causes rapid c
hanges in the littoral benthos and vegetation. The scale of the harmfu
l effects depends on both the range of regulated water level fluctuati
ons and the water quality; clear water lakes are more resistant to wat
er level regulation than humic lakes, The regulated lakes in northern
Finland were subjected to intensive ecological research during the 198
0s. Most of the studies were conducted by the water authorities and po
wer companies. As a result of these studies, the principles of so-call
ed 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. T
he ERP offers a simple way to illustrate to the regulation-permit owne
rs differences between various regulation practices. In the first case
study on Lake Kostonjarvi, 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. Th
e ERP was successfully put into effect with the exception of 1994, alt
hough the annual mean loss for hydropower production was US $0.14 mill
ion, In the second case study on Lake Oulujarvi, 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 mill
ion) a new plan with a lower summertime water level target (122.5 m) a
nd other complementary rehabilitation methods, including removal of sh
ore vegetation, was applied in Oulujarvi.