THE RIVER’S SEDIMENT IN THE NORTH-WEST RUSSIAN FEDERATION

Large forested areas, small slopes of catchments, the presence of lakes and swamps, the karstification of the basin, and a large share of underground river feeding contribute to a decrease in the amount of sediment runoff in the rivers located in the North-West of Russia. The distribution of sediment runoff during the year is determined by the flow regime of the river. The maximum values of the monthly solid runoff modules of all the studied rivers are observed at high water in March – May, usually reaching the peak in April, which is typical for the north-western region. Moreover, the passage of the maximum for the northern rivers (Pasha, Oyat and Tikhvinka) mainly falls on April – May; for rivers flowing in the southern part of the region – on March – April. The minimum values of the annual runoff modulus in the low-water period are extremely insignificant, especially in winter, and for some years they are close to zero. This is quite understandable due to the significant contribution of soil erosion to the formation of solid runoff of watercourses, with the most intensive processes of soil erosion occurring during the period of active snowmelt and floods. A steady increase in air temperature and precipitation layers for the North-West of the Russian Federation in the last thirty years in comparison with the previous climatic period has led to an increase in the river runoff for the studied watercourses. At the same time, according to the observations for the selected climatic periods, there is a decrease in the annual solid runoff. This can be explained by socio-economic changes – a significant decrease in agricultural activity in the region. In addition, taking into account the increase in the water content of the rivers, the decrease in the solid runoff can be explained by the redistribution of the intra-annual course of meteorological values that occurred as a result of climatic changes.