Regime of karst and glacial lakes at the carboniferous plateau of Tikhvin Ridge in the Northwest of the Russian platform

The studied territory is restricted to the central part of Tikhvin Ridge composed of karstified terrigenous and carbonate deposits of Carboniferous age. The central part of the area is a flat watershed dividing the rivers flowing northwest to Lake Ladoga and those flowing southeast to the Rybinsk water reservoir. The considered territory lies within the Msta River basin subjected to the recent tectonic uplift, forming landforms of Alpine type. Considerable longitudinal sloping of rivers provide intense surface and subsurface runoff (the active hydrodynamic regime). In the considered territory, the lakes are known with their areas changing significantly and quickly enough (months or years). The water in lake basins may disappear or fill them back completely. According to observations, these abrupt decreases and rises in water level do not correlate with seasonal floods, high water and low water periods. The phenomenon of drying and (what is particularly interesting) subsequent filling of lake basins with water is obviously related to the karstic groundwater regime, which is of scientific interest. The up-to-date research data include state geological maps (scale 1:200 000); topographic maps (scale 1:100 000); space images of the free-access Google Planet Earth Internet service, taken in summer and autumn 1984-2016; and weather service data. Cartographic materials were processed in GIS. The analysis of collected data allowed us to distinguish between two types of lakes, i.e., those with insignificant and substantial changes in area with time. Water reservoirs of the first type located on water-permeable moraine Quaternary deposits are classified as glacial. Other lakes overlying immediately Carboniferous deposits, have funnels in the basin bottom and are considered as karstic. The areas of two glacial and three karstic lakes have been measured (by interpreting satellite images) within the central part of the Carboniferous plateau at the Tikhvin Ridge. Variation in the glacial lake areas in the span of 33 years were found to be not crucial, being directly related to annual atmospheric precipitation not correlating with the changing areas of karstic lakes. Karstic lakes reveal 2-, 3- and 12-year-long cycles of significant changes in areas. These changes show decreases and rises in their levels and volumes. They are correlated among all karstic lakes irrespective of the average annual volume of atmospheric precipitation. It points to the significant participation of groundwater in alimentation of karstic lakes belonging to a single karstic water-bearing complex of underlying Carboniferous deposits. Authors relate the recurrent shallowing and filling of karstic lakes to the modern alternating tectonic movements leading to variation in permeability of karstic groundwater-bearing deposits.