Data collected between 1982 and 1985 from Lagrangian drifter experiments in which about ten drifters were tracked for ten to twelve hours from their release across the Niagara River mouth, and from concurrently taken ship-based temperature soundings at fixed grid stations off the river mouth, are used to develop a conceptual model of the mixing characteristics of the Niagara River in Lake Ontario. The data obtained suggest a three-stage mixing process. In the initial stage, the river inflow momentum dominates and the plume is well mixed vertically. In the intermediate stage, the interaction of the well mixed, buoyant river plume with colder water from deeper depths of the lake generates a sharp thermal front. In the final stage, the river plume responds to the prevailing winds and the general circulation of the lake. The correlation between these observed plume characteristics and the distribution of toxic contaminants such as mercury and mirex in Lake Ontario sediments attributed to Niagara River outflow is illustrated.