Fluctuations of Nero Lake in the Holocene

The paleohydrological settings in the Rostov lowland (Yaroslavl oblast) have been the subject of long-standing of disputes. The concepts for the Holocene fluctuations in the level of Nero Lake differ considerably among researchers. We have studied the structure of bottom sediments and bottom relief in the deepest, northeastern, part of the lake. A bathymetric survey was carried out. Drilling with the selection of undisturbed columns, GPR profiling, radiocarbon dating, and lithological analyses were conducted. Stratigraphic unconformities in the structure of the bottom sediments indicate a drop in the lake level in the Late Glacial and Early Holocene. The level dropped to 87 m asl, which is 7 m lower than the current water level in the lake. The size of the lake at this stage was reduced several times. The transgressive stage was established from 9 to 6.5 ka B.P.: the mean annual level of the lake could have risen up to 91–94 m asl, which is close to the modern level. In the interval from 6.5 to 2.4 ka B.P., a decrease in the level by 1–3 m below the current level is revealed. Then the level begins to grow gradually until it reached the current values 300–500 years ago. The main factor in the fluctuations in the level of Nero Lake in the Holocene is the change in the height of runoff threshold, caused by vertical deformations of the Ustye, Veksa, and Kotorosl river systems. These deformations were associated both with regional changes in fluvial activity and with the self-development of river channels.