IW5 Regulated watercourses

>> Background information

Development

Large scale lake regulation began in Finland during the 1920s. The area of regulated lakes increased rapidly during 1950?1970 when most regulation schemes were initiated. By the year 1979 approximately 1 240 000 hectares of lakes were already regulated. During the 1980s the area of regulated lakes increased by almost 80 000 hectares, but since 1990 new regulation schemes have not been established. The main reasons for lake regulation are water power production and flood control.

Annual regulation intensity varies from 10?20 cm to more than 10 m. Almost half of the total regulated lake area experiences a relatively weak water level fluctuation of less than one metre. In another 40% of the area the fluctuation ranges between one and three meters. Regulation intensity is more than three metres in 10% of the regulated lake area.

Impact on biodiversity

Water level regulation always has an impact on lake habitats. The extent of the impact depends on both the intensity and timing of regulation as well as the characteristics of the water body. Crucial factors include the shape and characteristics of the species assemblages of the water body. On gentle slope shores even small changes may have large scale effects.

Regulation results in declining water levels during winter. This is most harmful in lakes that are inhabited by White Fish, which spawn in shallow waters in autumn, and species which are sensitive to freezing. These species include for example Lake Quillwort and many benthic species. In regulated lakes the natural flood peak in springtime is lower. This harms spring spawning fish species, since they may not be able to reach the best spawning sites. Delayed rise of the water level during summer causes changes in the littoral zone and also poses a threat to birds nesting near the water's edge.

The ability to resist the effects of water regulation depends in part on the colour of the water. In dark waters the productive zone is narrow since less light can reach subsurface water colums. Under these circumstances even a small decline in water level during winter may result in freezing of the whole productive zone. Eutrophication sometimes reduces the damaging effects of regulation on the productive littoral zone, but lowering water level also increases the risk of oxygen depletion in eutrophic lakes during winter.