The construction of the Merikoski power plant in the 1940s closed the lower part of the Oulujoki river catchment area with a dam. The construction of the hydroelectric power plant had a significant impact on aquatic organisms and especially on migratory fish. The Merikoski dam obstructed fish migration to the Oulujoki river and resulted in the destruction of the breeding grounds upstream during its construction. Flow regulation and alterations affect ecosystems by limiting the natural fluctuations in water levels.