CHANNEL MORPHOLOGY AND RIVER FLOW IN NORTHERN EURASIA DURING THE LATE GLACIAL AND THE HOLOCENE

The relicts of extralarge palaeochannels are found throughout the periglacial zone of the last (Valdai, Weichsel) glaciation. Channel widths of macromeanders can be 15 times larger than the modern meanders of the same rivers. The closest modern analogues to these rivers are found in tundra of the northeastern Russian Plain and on the Yamal Peninsula. The hydrological conditions in the periglacial zone of northern Russia were characterized by spring floods with levels much higher than today. Annual discharges also exceeded recent discharges due to higher winter and spring precipitation, and only in rare cases due to glacial melt water input. The annual discharge of the Severnaya Dvina River was close to the modern one, although she was fed by a smaller catchment area (68% of the modern one). The lower part of the Severnaya Dvina basin was occupied by an ice-dammed lake. The annual discharge from the Mezen' River was 225% of the modern one, and from the Pechora River 175% of the modern value. The combined influence of greater flow and higher discharges caused the formation of very large river channels with macromeanders found now on low river terraces and floodplains. These palaeochannels can serve as a tool for geomorphic and stratigraphic correlation of alluvial and terrace units in river valleys. For central Russia (rivers Moscow, Protva, Seim, Khopyor) this the time span of 16 to 11 ¹⁴C kyr B.P. In northern Russian Plain ¹⁴C dates are lacking for the «stage of macromeanders».

During the transition to the Holocene degradation of permafrost and increasing soil permeability in the spring caused a decrease in runoff coefficients, flood flow and maximum discharge for the snow thaw period. Changes in ground water regime during the summer caused an increase in the basic flow and forestation of floodplains and sand bars. Large periglacial channels were abandoned, transformed into oxbow lakes and bogs. The Holocene channels and meander belts were more narrow than the periglacial ones. The degree of channel metamorphosis was significantly different in various parts of northern Eurasia, because of non-uniform discharge changes during the Late Valdai/Holocene transition. The rivers of the tundra zone are still in “periglacial” conditions, and the modern river flow is close to periglacial. In other zones the present annual flow is lower than in the Late Glacial. For the taiga zone of the Russian Plain it is 80-85% of the “periglacial” values in the east and 30-60% in the west, in the deciduous forest zone - 40-50% in the east and 20-25% in the west, in the steppe and forest steppe zones - 40-60% in the east and nearly 10% in the west of the region. Presumably, the main cause of the different degree of changes in the annual flow was the spatial variability in decrease of the flow coefficient at the Late Valdai/Holocene transition.

The overall hydrological changes were characterized by decreasing spring and annual flow in the early and increasing flow in the late Holocene. In the northern and central Russian Plain these changes were similar but not synchronous. The driest period was the Early Atlantic in the north (River Vychegda) and Late Atlantic in the center of the Plain (the Oka River basin). One more episode of low floods occurred in the central region during the Medieval Warm Period (IX-XII с AD), when permanent settlements existed on floodplains in the upper Dnieper and Oka basins. The analog of this period has not yet been detected in the north of the Plain. The last millennium is characterized by high river discharges all over the Russian Plain with maximum flood activity during the Little Ice Age (XVIII-XIX c).

This study was supported by the Russian Foundation for Basic Research, Project 06-05-65218.

Reference:

Sidorchuk A.Yu., Panin A.V., Borisova O.K. Channel morphology and river flow in northern Eurasia during the Late Glacial and the Holocene. Correlation of Pleistocene Events in the Russian North. International Workshop Abstracts. 4-6 December 2006. Saint-Petersburg, 2006, p. 88.