Environmental factors related to water level regulation — a comparative study in northern Finland

The environmental conditions of the littoral zone were studied in the regulated Lake Ontojärvi and the unregulated Lake Lentua in northern Finland. The general aims of the study were to analyse the environmental factors related to water level regulation in the littoral zone and to produce information for assessing the effects of hydroelectric development in northern lakes. The study was basically carried out by comparing the littoral environments of the two study lakes. The most visible effects of water level regulation were related to the raised water level, which yielded erosion of sandy shores at the beginning of the regulation. Another effect of lake regulation was the altered fluctuation of the water level, which led to bottom instability and increased the size of the frozen and ice penetration zones. The effect of ice penetration was also easy to recognize on the shores of Lake Ontojärvi, where the surface sediment was frozen to a greater depth and across wider areas than in Lake Lentua. Below the freezing zone, the ice just pressed down on the sediment. The shores of Lake Ontojärvi were steeper than those of Lake Lentua, what affected the distribution of bottom types, with sandy bottoms being more common in Lake Lentua than in Lake Ontojärvi. The factors related to site exposure included effective fetch and the shape of the shoreline. The sedimentation level correlated only with the slope and was not predicted by the fetch or shape. The vertical reduction of light was estimated on the basis of water colour. The main environmental factors from the two lakes were used in a discriminant analysis to predict the bottom type distribution of the littoral (r2 = 0.41).