Based on the research of the chemical erosion intensity in the upper course of Tamnava, the appearance of its negative value has been found. Further researches, that enclose the lower course, confirmed this fact. This research opened the important question about the causes of this phenomena. Analytical method eliminated all expected causes and led to conclusion that the reason for this should be found by using the more precise research of the mineralisation of the precipitations. It was necessary to continue the measurings of the precipitation mineralisation in the greater number of locations and in the wider time period, as well as in the different meteorological conditions, and after that to establish the subordination between the amount of precipitations and the entered TDS. Only after these additive researches we could be able to find out the relation between ''entry'' and ''exit'' and to find out the intensity of the chemical erosion in this, and in the other river basins. For now, we are able only to find out the carrying of TDS, as well as the reduction of the chemical evacuation for the non - carbonated part of the basin (corrected value).