Anthropogenic degradation of vegetation leads to a reduction in the hydroclimatic and protective functions of forest cover on the catchment and in the river channel. This results in an increase in content of small fractions in channel deposits that serve as spawning areas for Pacific salmons, which affects negatively their survival during the embryonic and larval period of development. The small mountain rivers in the southern part of Sakhalin were used as an example in investigating the influence of vegetation growing within the basins, on the content level of small-gravel, sand and aleurite fractions in channel deposits in the river sections preferred for spawning by pink salmon, the main species of Pacific salmons. For this purpose, relationships were identified between different characteristics of forests on the territories at an identical distance from the channel and the content of particles less than 3 mm in diameter in river alluvium. It is shown that with an increase in anthropogenic degradation of vegetation cover, the content of small fractions in earth materials increases. The main influence on the content of particles smaller than 1 mm is exerted by the forest belt 100 m in width, adjacent to the bank line (on either side of the river). Their content is lower in areas surrounded by dark-coniferous forests.Keywords: granulometry, protective role of forest, forest cover, small rivers, Sakhalin, spawning.