О СОВРЕМЕННОЙ ДИНАМИКЕ ПРОДОЛЬНОГО ПРОФИЛЯ СРЕДНЕГО ТЕЧЕНИЯ РЕКИ ЛЕНЫ

Abstract

The history of fluvial sedimentation in the middle Lena River valley in Central Yakutia in the Late Pleistocene and Holocene is revisited. It is shown that the alluvial cover of the Lena-Taatta interfluve was accumulating almost uninterruptedly throughout the Pleistocene, with incision events corresponding to major glacial epochs – Samarovo and Zyryan glaciations. The stages of incision and aggradation with the predominance of the latter alternated in the middle Lena River during the Holocene. Directed river incision is not typical for the middle Lena River neither in present nor in most periods of the Neopleistocene. The presupposed features of the modern incision are considered, and it is shown that such features are not manifested in the dynamics of the longitudinal profile and the structure of the bottom of the middle Lena River valley. The downward shift of the stage-discharge curve on the hydrological station of Tabaga, 40 km upstream from Yakutsk, is due to its enhancement by the Yakut Department of Hydrometeorology and Environmental Monitoring, but not because of hydro-morphological processes. Certain features of the structure of the riverbed and the valley of the middle Lena River, typical for the incising rivers, may be associated with modern cryogenic processes. In the process of syngenetic freezing of floodplain sediments, the certain floodplain levels can increase their relative elevation, which contributes to the floodplain consolidation. Floodplain levels can be formed due to differences in the lithology of sediments with different heaving potential. Riverside thermo-erosion associated with the thermal influence of river flow and the destructive action of the ice, erodes the frozen island front faster, than outside the permafrost area. As a result, the islands are displaced downstream, but in their upper part, there are sandbanks indicating the aggradation of sediments. Thus, there are no signs of the modern incision trend in the relief of the middle Lena River territory. Riverbed sections with predominance of local erosion and aggradation are alternated, which is primarily due to the hydraulic structure of the flow, as well as the influence of neotectonic movements.