Hydrology of Lake Atnsjoen and River Atna

The hydrological investigations in the Atna basin started in 1917 with discharge measurements at the outlet of Lake Atnsjoen. Ice data are available from the lake since 1950 and water temperature data from 1980. With the start of the FORSKREF program in 1986 the number of hydrological stations increased substantially and included also snow measurements. During the FORSKREF period 1986-98 the annual mean discharge was 8% lower than during the normal period 1931-60. The annual flood is highest during the snow melt in spring, an exception was 1987 when a rain flood in October was the annual high. The spring flood in early June 1995 was the largest ever recorded and had a 100-200 year return period. The water temperature is low above tree-line at 1000 m a. s. l., rarely exceeding 7-8. C. The temperature increases gradually with decreasing altitude and is periodically above 15degreesC at 400 m a. s. l. The Atna Lake is an important heat source for the river downstream. In this lake, wind mixing normally prevents the development of a strong summer thermocline. The lake usually freezes over in late November and has a stable ice cover of 50-90 cm thickness in early spring. The ice normally breaks up in late May; thus the mean ice-covered period is 6 months. The ice cover on the river is less stable and rather large ice-runs are common on the lower part of River Atna. The snow measurements are carried out in April in a small subbasin between 800 and 1200 m a. s. l. The snow cover is unevenly distributed due to strong wind transport, but the pattern is more or less the same from one season to another. The mean snow accumulation in this basin is equivalent to 205 mm of precipitation.