Current dynamics of fluvio-lacustrine systems of Onon-Torei High Plain (Southern Transbaikalia)

The article shows the peculiarity of functioning conditions of fluvio-lacustrine systems of the Onon-Torei Plain, located at the southern boundary of the permafrost zone and along the northern boundary of the vast arid areas of Central Asia, as well as at the junction of the Upper Amur basin with an area of internal drainage. In these conditions the fluvio-lacustrine systems are highly dynamic. The course of exogenous relief-forming processes in time, and their interaction and interchange are considered; mechanisms of modern functioning of fluvio-lacustrine systems are discovered. The long term cycles of relief reshaping lasting for 27-35 years that include dynamic phases with different range of processes, direction, distance and intensity of substances transfer, are determined. The phases successively alternate in time and are invariant of systems functioning. In the normal zonal phase the near transport of matter conducted by slope processes of moderate intensity is observed, which results in the integration of matter in lake basins, the bottoms of canyons and river valleys. In the extremely wet transgressive phase the far fluvial transport of matter in the Upper Amur basin or stagnant lake basins dominates. Basin erosion is enhanced. Suspended sediment runoff module of the Borzya River increased by 15 times compared to the long-term average. Activation of gully erosion is characteristic for the phase, in the dynamics of the relief the role of cryogenic processes also increases dramatically. This increases the number of blow up and aufeis formation sites, expanding their area. In winter, the river channels form a chain of ice mounds, whose formation and destruction is accompanied by channel sediment transport. During extremely low humidity and lakes regression aeolian processes of high intensity dominate. Windward edges of coastal river benches are subjected to significant aeolian processing. On the surface of the river and lake terraces deflation basins are highly developed. Bottom sediments of lake bottoms dry baths are carried by the wind in a south-easterly direction to the stagnant areas of Mongolia. A tendency of expansion of Uldza-Torey endorheic basin that occurs on a background of aridization of the area under study over the past two thousand years should be considered in the forward estimates of changes in the relief area. Due to the fact that the intensity of aeolian migration of the substance is higher than the module flow of suspended sediments, the main stream of matter within the plain goes from the Onon basin to the internal drain. The boundaries of the individual lake basins and endorheic lake basins are shifted mainly to the north.

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