Downstream wetlands play an important role as an interface between terrestrial and coastal areas, and nutrients and dissolved organic matter in downstream rivers are recognized as important parameters for various ecosystem functions and services in the watershed ecosystem. In this study, nitrate nitrogen and dissolved organic carbon in river water were observed with high temporal resolution at 10-minute intervals using water quality sensors in the downstream wetland area of the Bekanbeushi River watershed in northern Japan. The river water flow was similarly measured to examine the relationship between water quality and riverine volume transport in downstream river water under the influence of tidal fluctuations. The tidally induced cycles of riverine volume transport (i.e., discharge (tidal ebb) and backflow (tidal flood)) had a nonnegligible effect on the concentration-flow (C-Q) relationships of nitrate nitrogen and dissolved organic carbon. Tidal saltwater upwelling was also found to disturb the C-Q relationship by increasing the nitrate-nitrogen concentrations and decreasing the dissolved organic carbon concentrations. Intense rainfall infiltration also contributed to the reduction in the concentrations of both components. The results suggest that changes in hydrologic processes due to tidal fluctuations and heavy rainfall have a nonnegligible effect on nutrient and dissolved organic matter concentrations in downstream watersheds.