Abstract. The hydrological and geomorphological processes occurring in the macrotidal Mezen and Kuloy estuaries are completely subject to the effects of the tidal wave. The main purpose of studying the dynamics of suspended matter and sedimentation processes is to determine the river and marine factors affecting sediment transport and the features of lithodynamic processes. Expeditionary studies of the Kuloy estuary, which were conducted by the author in 2005-2019s and the summer of 2022, showed that a high concentration of suspended sediments compared to the adjacent areas of the river and sea is observed in areas of reversible currents. The concentration of sediments was determined by weighting of the filtered samples taken during the tidal cycle, while simultaneously recording the hydraulic parameters of the water flow. The maximum value of spring tides in the of Kuloy is 10 m, in the neap tides – 4.8 m. Tidal velocities during of the spring tides reach 2.5 m/s. The salinity of water in the Kuloy estuary varies from 3.5 to 24.0 ups. The greatest values of turbidity are noted in the lower part of the estuaries during the low water and in the top of the studied estuaries in the high water. The maximum turbidity in the Kuloy estuary reaches 8.7 kg/m3 during the low water due to the concentration of suspended sediments in the bottom horizon. The turbidity of water in the surface horizon is stored in the range of 0.05–0.10 kg/m3. The increase in turbidity is facilitated by wind wave on the offshore and intense abrasion of the banks in the estuaries. The conditions for the economic use of the mouths of the Kuloy river depend on the tides. Navigation can be carried out only during the period of high water, and requires taking into account the dynamics of the fairway.

Keywords: estuaries, hydrological regime, water mixing zone, suspended sediments, maximum turbidity, erosion-accumulation processes, tidal flat