The aim of the article is to assess the average long-term background hydrochemical input of chemical elements and organic matter from the flows of the natural component of the environment "Rivers" flowing into the South, Selenginsky, Middle and North reservoirs of Lake. Baikal. The results of calculation of the average annual amount (g/year) of chemical elements and organic matter (Na+, K+, Ca2+, Mg2+, Al, Si, Mn2+, Feобщ , SO42-, HCO3-, Cl-, NO3-, PO43-, Cr, Cu, Cd, Hg, Pb, Sr, Zn, Co, U, V,

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... , Cорг, Nорг, Pорг, Sорг, CO2, Ti) in the streams of the natural component of the environment "Rivers" flowing into the South, Selenginsky, Sredniy, Severny reservoirs of Lake Baikal are presented. The scale of the river chemical input into the reservoirs and the contribution of the "River" flows to the chemical balances of these reservoirs have been determined. It has been established that only in the Selenga reservoir the rivers carry a significant amount of matter (about 3%). The contribution of river flows to the chemical balance of the reservoir is 3.54; 5.4; 17.5 and 21.5% in the South, Selenga, Middle and North reservoirs, respectively. The rivers flowing into the Selenga reservoir carry 70.6% of the total amount of matter brought by the rivers into the lake. Whereas the rivers of the Northern, Middle and Southern reservoirs carry 14.0; 11.6 and 3.8% of the matter. Only in the Selenga and Northern reservoirs, the rivers (tributaries) are the main sources of macrocomponents (K+, Na+, Ca2+, SO42-, CO3-, Cl-), a number of microcomponents (Rb, Mo, Hg, Sr, and Cu and Zn in the Selenga river-), organic matter (Corgг, Norg, Porg), and biogenic components in the Selenga reservoir (NO3-).