Regulated watercourses

While many rivers in Finland have been dammed and a number of lakes are regulated, regulation does not affect all water areas.

Dam structures used to regulate river basins come in many types from low bottom weirs to large power plant dams made from concrete. Dams can be used to regulate both water levels in lakes and flow rates in rivers.

Around one third of Finnish lakes’ surface area is regulated. A regulating dam, or power plant dam, is placed at the outlet of a regulated lake at the start of the river along which the lake’s waters discharge. There are over three hundred regulated lakes in Finland, including Päijänne, Näsijärvi and Inarijärvi.

Among the great rivers of Finland, the rivers Simojoki and Ounasjoki as well as the transboundary rivers Torne and Tana still flow free. While Lake Saimaa is not regulated, its water levels fluctuate naturally. After a particularly dry period or spell of heavy rains, however, more or less water than usual can exceptionally be run from Lake Saimaa into the Vuoksi. This prevents harmful water level fluctuations which might cause flood damage or, for example, put the breeding of the Saimaa Ringed Seal at risk.