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6.3.3.      Case: Regulated water levels of lakes

Regulated lakes in the Nordic countries

Source: Martunnen et al. 2006: 

There are hundreds of regulated lakes in Finland, Norway and Sweden (Table 2). For instance, in Sweden, there is 563 lakes larger than 1 km2 with water level regulation vary from 0.1 m to 35 meters. In Norway, there are approximately 800 reservoirs registered in NVE’s database, and a further 100 are assumed to exist without being registered so far. In half of these reservoirs, the water level fluctuation is more than 5 metres. The highest regulation amplitude is 140 m. In Finland, the water levels from 100 regulation projects of the total 350 projects have been analysed. Finnish regulations are usually relatively mild in terms of annual water level fluctuation. Half of these projects show that the annual water level fluctuation is less than 1 metre. The maximum water level fluctuation in the most heavily regulated lake in Finland is 7 metres.

Relative proportions of regulated lakes to the total number of lakes is the lowest in Finland (8 %) and the highest in Scotland (46 %), where the combination of high altitude and high precipitation favours establishment of reservoirs. However, in Finland, most of the largest lakes are regulated and consequently one third of the total lake area (about 11 000 km2) is regulated.

In summary, many Swedish and Norwegian reservoirs are much more heavily regulated than Finnish ones. However, the regulation amplitude itself does not directly describe the magnitude of ecological impacts of regulation. For instance, in Finland lakes are generally much shallower and their water is more coloured and consequently the productive zone is narrower than in Norwegian and Swedish lakes. Furthermore, there is a big difference in the use of regulated watercourses between Finland, Sweden and Norway. In Sweden and Norway most reservoirs are located in remote areas where recreational use of the watercourse is usually of minor importance, whereas in Finland the regulated lakes are almost always important for recreational purposes.