THE WATER LEVEL REGIME OF LAKE ILMEN

Abstract

Researches of changes in the water level of Lake Ilmen are important for studying the development of the river network in its basin, since it is the basis of erosion for them. The purpose of the work was to assess the level regime of Lake Ilmen during the Holocene, including the modern period. The level regime of the lake is determined not only by the inflow of waters from the catchment, but is also regulated by the runoff of the Volkhov River flowing out of it, which, which prior to the construction of the Volkhov Hydroelectric Power Station in 1926, depended on the marks of the Pchevsky and Veletsky rapids in the downstream. During the Holocene, the marks of the Pchevsky and Veletsky rapids were decreasing, because they been eroded by the Volkhov River. An approximate reconstruction of the change in rapids marks has been carried out, depending on the humidity of the climate in previous centuries. Evaluation of a varying degree humidification over a century / millennium is rather arbitrary and was taken as the ratio of the number of rainy years to years with droughts based on annals data. By the beginning of our era, the minimum water level of the lake was not less than 19.5 m. The maximum water level most likely did not exceeding the mark of 24.5 m, considering the similarity of climate to the last centuries, that is, the amplitude of the water levels was less than modern. Only climatic features determined the water level regime of the lake starting from the second half of the first millennium to the present day. On the grounds of the fact that the minimum bottom marks of some rivers, flowing into lake Ilmen (in particular Lovat’, Msta and Shelon’), are lower not only than the minimum water level of the lake, but also than the minimum marks of its bottom, we can do a preliminary conclusion that the water level of Lake Ilmen in the past was rather lower than at present and was at modern mark of 16-17 m Baltic system.