Abstract

The Middle Angara region is distinguished by a complex structure of geosystems within the river basins, which is a consequence of the orography, the varied natural conditions, and of the characteristic properties of anthropogenic and technogenic influences. The upper part of the river basins of the Angara tributaries is characterized by dark-coniferous native and arbitrarily native complexes, the middle part of the basins is dominated by light-coniferous steppe-grown southern-taiga landscapes, and meadow-steppe areas with birch tree stands and shrubs and steppe forbs-grass areas with the inclusion of halophyte-meadow biocenoses prevail around the outfall. The present secondary-derivative landscapes (residential, pasture, line-and-road, and pyrogenic) occur in the lower parts of the basins and shores of the Bratsk reservoir.

In the territory of the basins, the spectrum of exogenous processes varies, which influences the amount of sediment runoff and the turbidity of flows. The upper part of the basins of the Angara tributaries is dominated by fluvial processes under humid morphoclimatic conditions, while the slopes show largely cryogenic processes and slow displacements of earth materials (decerption, and defluxion). The middle part includes a transition zone, while the lower part of the basins (outfall), and the territory along the parent river are characterized by a sharply continental Angara-Lena morphoclimatic type of morphogenesis; water erosion and accumulation, and Aeolian, karst, landslide and other processes occur in combination. In the upper part of the basins, the modulus of sediment runoff is 1.6 t/km² per year, and the yearly mean turbidity is 10-25 g/m³. In the lower part, these figures increase to 15-25 t/km² and 100-150 g/m³, respectively. The yearly mean turbidity of the water streams exceeds the zonal value by a factor of 3-5. The Middle Angara region is distinguished by a high anthropogenic load – according to the water pollution index, the Angara is classed as moderately polluted and polluted. There is a need for a geomonitoring of the seasonal and annual dynamics of erosional-accumulative relief-formation in the river basin.

Keywords: water and sediment runoff, structure of geosystems, natural landscapes, anthropogenically modified landscapes, fluvial processes, gully erosion.