The Riverine Past of Lake Seliger

Lake Seliger has been always classified as a former glacial water body, a remnant of a vast proglacial lake, which has existed since the disappearance of the last glacial Ice Sheet in the territory 18–19 thousand years ago. The study was focused on areas of southern pools of the lake looking like underflooded meandering river channels. Bottom sediments were drilled in three such areas and ground penetrating radar profiling was carried out on two areas. Lake silt, several meters thick, was found to overlay coarse sand, which is most likely of riverine origin. A feature in favor of this is a characteristic asymmetrical bed profile in the supposed parts of ancient river bends and the time of transition from active flow to the regime of stagnant water body since 14.5 thousand years ago: in that time, glacier edge already lied far from here, and glacial melt water did not penetrate into this area. The inundation of segments of river valleys and their transformation into lake pools was provoked by damming of the Selizharovka River, which originally flowed out from Lake Seliger near Ostashkov Town. About 14.5 thousand years ago, the rapidly growing delta of the Krapivenka River, a left tributary of the Selizharovka, started growing rapidly, damming the latter river. In the period of 2 thousand years, the damming effect extended over the entire 15-km segment of the valley and reached the source of the river in Lake Seliger. In the Holocene, the level rise continued, though at a lesser rate. The total rise of lake level was 7–8 m, which caused flooding of the lower reaches of the valleys of rivers emptying into the lake and transformed them into estuaries. It is shown that glacioisostatic deformations of the Earth crust could not reverse the flow from Seliger into the Baltic Basin. After the disappearance of the most recent glacier, Lake Seliger and its tributaries have always belonged to the Volga Basin; this is to be taken into account in the reconstruction of the Volga runoff and the water balance of the receiving water body (the Caspian Sea).