The Luodonjarvi bay was cut off from the northern part of the Bothnian Bay (Baltic Sea) in 1962 by a dam to provide a supply of freshwater. The bay is fed by river water from small rivers. Monitoring results have indicated a rather fast (ca 2 years) change from brackish to freshwater. According to sediment diatom analyses, the process has lasted much longer. The northern part of the reservoir is affected by acid river waters, but some of the original brackish water is still there. The freshening process is described using different ecological groupings of diatoms.