We report results from hydrochemical investigations into the overland runoff within the watershed basin of the Tugursky Gulf in the southern part of the Sea of Okhotsk, in the area of the prospective construction of the tidal power plant. We examine the spatio-temporal dynamics of hydrochemical parameters of the tributaries flowing into the gulf. An estimate is made of the runoff of dissolved substances. The study revealed a significant influence of the entry of dissolved organic substances from swamped territories on the southern part of the gulf’s water area. The water exchange with the sea and its temporal variability are responsible for the distinctive features of chemical composition of the river waters in the confluence zones.Keywords: Tugursky Gulf, background concentrations, ionic runoff, mixing zone, salinity.