Understanding the functioning of the aquatic ecosystem within the mainstem Athabasca River is of paramount importance if protection against the input of contaminants from oil sands developments is to be afforded to the Athabasca River itself, the Peace-Athabasca Delta, and Lake Athabasca. The term assimilative capacity has been applied to denote the dynamic ability of aquatic ecosystems to remain viable and productive in the face of external factors (natural or anthropogenic). An implicit assumption within this definition is the fact that aquatic ecosystems possess the ability to change in response to external factors while maintaining their productivity and diversity. The rate and extent of this adaptive capability is the underlying process of assimilative capacity which must be understood and therefore examined.