This paper presents the seasonal and annual variations of the physical parameters, nutrients, major ions and trace metals in the Niagara River for a period of 15 years (1975–89). The discharge during this period was found to be increasing until 1986, after which a large decrease was observed. As a consequence, the specific conductance decreased until 1985 due to the higher dilution effect as well as the lower concentrations of chloride and sodium. High concentrations of total phosphorus during the winter months corresponded with high turbidity and high suspended solids concentrations. In addition, a steady decrease in total phosphorus concentrations was observed until 1984, but since then has levelled off. Nitrate nitrogen concentrations were found to be steadily increasing from 1975 to 1987, and then a large decrease was observed. Both chloride and sodium concentrations decreased substantially from 1975 to 1989, as a result of upstream industrial input decreases, but concentrations since then have levelled off. Iron and aluminum concentrations and loads have shown steady increases since 1975, probably due to increased sediment loads in the October to January period, while lead, nickel, zinc and chromium concentrations and loads are declining.