Quantitative distribution and grain size composition of suspended particulate matter (SPM) in the marginal filter of the North (Severnaya) Dvina River during summer low water periods of 2001-2005 were analyzed in water on board immediately after sampling (without preliminary treatment) using a Coulter counter. This analysis revealed main regularities in transformation of grain size spectra at successive salinity steps of the marginal filter, as well as boundaries between these steps based on data obtained by direct complex studies of SPM dispersion. It is established that water salinity is the main factor that controls changes in grain size distribution and composition of suspended matter in the marginal filter. Concentrations of <0.01 mm size fraction and salinity demonstrate negative correlations between each other. It is shown that areas characterized by mass development of phytoplankton are located along the outer boundary of the marginal filter (at the biological step), where salinity reaches 23-24 psu. Contents of particulate forms of some chemical (lithogenic) elements and organic carbon indicating genetic composition of SPM and their relations with grain size composition of SPM are studied
