Despite their protected status, aquatic ecosystems of Banff National Park have been subjected to a number of human stresses. Largely as the result of stocking programs earlier in the century, 10 species of nonnative fishes now occur in the Park, while one endemic subspecies of fish has been extirpated, and 2 other species are threatened. A number of rare invertebrates occur in hot springs and caves, including one mollusk that is endangered. Key invertebrates were extirpated from a number of fishless lakes by stocked fish, and in some cases have not returned, even though fishes did not survive. Restoration efforts in 2 small alpine lakes are described. Addition of nutrients and road salt have changed the chemical nature of the Bow River and its tributaries, and caused incidence of benthic algal mats to form in some sections. Impoundment and diversions affect over 40% of the Bow River catchment within the Park. Airborne organic contaminants concentrate in glaciers and high elevation snowpacks, yielding amounts high enough to contaminate fisheries to levels that in some cases approach guidelines for human consumption.