Natural and anthropogenic factors of small riverbed's formation of the Tunka depression (Republic of Buryatia)

This article discusses the influence of natural and anthropogenic factors on the formation of small riverbed using the example of the Tunka River Basin located within the Tunka depression in the western part of the Baikal rift (51°45'27.1"N; 102°30'05.6"E). At present, the factors of small riverbeds formation haven't been practically studied in mountain depression conditions. Analysis of field, hydrological, cartographic, geomorphological data, as well as remote sensing data showed that, among the main factors of natural origin, the greatest influence on riverbeds are geological structure of the territory, relief and climatic conditions. The tectonic conditions of the territory, namely, sections presence of lowering the blocks of the earth's crust in the central part of the basin, are the cause of development of tortuosity and multi-channel riversbed (Fig. 3). Relief and climatic conditions affect the incision of rivers in mountainous part of the study area, while the maximum precipitation that falls in July-August causes summer floods (Fig. 2). Shortterm floods contribute to development of dangerous geomorphological processes, for example, mudflows. Mudflows lead to blockage of valleys by mudflow deposits and restructuring of the river network, and also intensify the processes of riverbed incision (Fig. 4). The shift of small riverbed on the slopes of Tunkinsky Goltsy to the west, as well as the presence of mudflow valleys, pose a great danger to Arshan, which was already exposed to negative consequences of the mudflow in 2014. The morphostructural plan of the territory determined variety of morphodynamic riverbed types: a wide-floodplain meandering multi-channel type within lowering of the blocks of the earth's crust, relatively rectilinear and incised types when crossing faults. The map-scheme of the river Tunka (Fig. 3) was created as a result of field data analysis, cartographic material and remote sensing data. The map shows the main morphodynamic types of small riverbed. The most common type is incised relatively straight riverbed l which indicate the rise of individual blocks of the earth's crust in the basin. Of all the types of economic activity, the most significant factor of direct impact on the morphodynamics of small riverbeds is land reclamation by draining. Land reclamation led to straightening of riverbeds sections of wide-floodplain type of small rivers (Fig. 5), thereby reducing the intensity of the processes leading to meandering of the riverbeds.

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