Abstract <p>The distribution of dissolved matter in the macrotidal Kuloy River estuary was investigated, and a comparative analysis of the results with research data on the Mezen River estuary with a similar hydrological regime was carried out. The dominant modification of the Kuloy River water mass, which is involved in mixing with seawater, was identified. It was shown that the similarity of the patterns of dissolved matter migration in the Kuloy and Mezen estuaries was inherent in (i) additional input of phosphates and silicon from pore water of regularly disturbed bottom sediments; (ii) enhanced desorption of barium from terrigenous matter in prolonged contact with saline water; (iii) removal of manganese, iron, lead, and hydrolysate elements (Al, Ti, Y, Zr, rare earth elements) during coagulation and flocculation of organic and organomineral colloids, and (iv) conservative distribution of the remaining considered elements (major ions, Li, Rb, Cs, Sr, F, B, V, As, Sb, Mo, W, Ni, Cu, Zn, Cd), with the exception of uranium. The main differences in the chemical transformation of river runoff are the extent of nonconservative behavior of chemical elements under the influence of various processes: in the Kuloy estuary, a more intensive input of nutrients from sediment pore water, a weaker barium desorption anomaly, and a conservative distribution of uranium in contrast to its desorption in the Mezen estuary were observed, as well as a greater effect of immobilization of hydrolysate elements.</p>