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This thesis examines the ecological consequences of sewage disposal in aquatic ecosystems. Sewage simultaneously represents a form of enrichment and a source of stress to a receiving system. Enrichment effects dominate when sewage loading does not exceed the capacity of the system to assimilate waste, as is presently the case in the St. Lawrence River. We developed a method to quantify the assimilation of sewage-derived organic matter by riverine biota, then used this method to examine the pathways by which sewage enhances secondary production in the St. Lawrence. We showed that the relative importance of dissolved nutrients and particulate organic matter to the river food web is to a large degree dictated by the physical and biological characteristics of the local environment. The effects of this enrichment on the receiving water community were also influenced by habitat characteristics. Analysis of body size distributions revealed that only the largest organisms had higher densities at enriched sites, and that the identity of these organisms depended on habitat structure. Densities of smaller organisms were controlled by a combination of habitat characteristics and feeding interactions. This pattern was consistent with food web models of top-down (consumer) control. At each trophic level, relatively invulnerable (large) prey achieved higher densities, whereas densities of more vulnerable (small) prey were controlled by their predators. Finally, we synthesized the ideas outlined above to predict how conditions in the St. Lawrence will change in the future as a result of lower water levels. More extensive macrophyte cover, slower current, and warmer temperatures in a shallower St. Lawrence will all enhance the ability of the system to physically retain and biologically process sewage nutrients, but will also increase the likelihood of negative effects such as anoxia.