ИСТИНА

Southern half of the Russian Plain is occupied by forest, forest-steppe and steppe landscape zones. Until the 17th century population density was low and area of cultivated lands was negligible. Area of arable lands increased considerably on the south of forest zone since the beginning of 18th century. Intensive cultivation of the forest-steppe and the northern part of steppe zones began in the end of 18th century. Only in the end of 19th century lands of dry steppe zones were cultivated with increasing area of arable lands from 5 to 65% for only 20 years. Some parts of dry steppe in the south-east of Russian Plain and on south of Western Siberia were intensively cultivated only in the middle of 20th century after World War II. So in case of Russian Plain it is possible to evaluate the antropogenic impact on landscape based on documentary evidence for region scale and in some case for local scale. Detail analysis of the temporal-spatial dynamics of stream net density (SND) for different landscape zones of the Russian Plain was undertaken using comparison of historical topographic maps for few time intervals since the end of 18th century. It was found that maximum SND decrease is observed at the northern parts of the steppe zone, where total SND in the middle of 20th century was 50-60% of those at the first quarter of the 19th century. It was confirmed that intensive small river aggradation was associated with period of intensive cultivation of basin area, which led to increasing of surface runoff and sediment transport from cultivated lands to the valley bottoms with proportional decreasing of underground runoff, feeding constant flow in the river channels during low water. The highest input of basin sediment to the river valley was coincided with periods of gully erosion intensification, which in turn were due to changes in land use (a sharp increase in arable land, plowing the steep valley sides, increasing the area of individual fields). It was determined that intensive small river aggradation is observed at the river basin with area of arable lands under 40-60% from total basin area. It is confirmed by high coefficient of correlation (r=0,79) between the area of arable lands and the depth of floodplain sediment accumulated since the beginning of intensive cultivation for individual river basin. Quantitative assessment of sediment redistribution within small catchments of the agricultural zones of the Russian Plain was made based on application of different methods and approaches. It was established that about 2-4 m of sediment was deposited in the valley bottoms in the upper chains of drainage net (river with length up to 10 km) since the beginning of intensive cultivation.