VARIABLITY OF VELOCITIES OF VERTICAL WATERCOURSE DEFORMATIONS OF PLAIN RIVERS UNDER CONDITIONS OF PRIILMEN LOWLAND (BY THE EXAMPLE OF PORUSIA RIVER)
The well-known patterns of regressive erosion from the mouth to the sources of watercourses on the rivers of Priilmen'ya are performed discretely, since the basis of erosion (first the level of Sredne-Lovatsy (Privaldaysky) glacial lake, and then Lake Ilmen) is decreased over 12.5 thousand years from elevations of about 85 m to modern 18 m, stabilizing for a long time at elevations of 60, 40, 30 m. Comparison of the longitudinal profile of the Porusia River constructed from cartographic materials with supreme erosion cut profile calculated according to the formula N.I. Makkaveev showed that the profile of this river has not yet been sufficiently developed, although the excess of the edge of the valley above the bottom of the river in its middle course is 10-15 m. The correctness of the calculation of the erosion curve is confirmed by its coincidence with the longitudinal profile of the river Lovat', which exists for a longer time and has the greatest water content among the rivers of the Priilmen lowland. According to topographic maps of 1932 and 1984, as well as our own measurements of 2016-2019 it was found that the total lowering of the river bottom near the village of Mintsevo (71 km from the source) over 90 years (1930-2019) reached 3.5 m. Using other assessment methods, including surveys of local residents, the depth of the erosion cut in this locality is approximately the same period of time ranged from 1 to 1.6-1.8 m. On a different locality of the river (53 km from the source) for a comparable period, similar rates of vertical channel deformations were obtained from cartographic materials. Studying of additional let make a hypothesis according to which over 300 years the bottom marks could decrease by 10 m. An analysis of the soil section data allowed us to conclude that the rate of vertical erosion had strongly marked discrete character. On hard washable areas, vertical erosion slowed sharply and was some thousands years, but for thick layers of sandy loam eroded for centuries.
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