increasing water contaminants has been a growing problem in southern ontario due to rapid urbanization, agricultural runoff and pollution. determining how riffles and pools respond to changing contaminants is crucial to understanding effects on species living within those particular habitats. sites were studied along sixteen-mile creek in halton, ontario, to compare water quality and fish occurrence. electrofishing was used to sample johnny darter, longnose dace, river chub and rainbow darter as models for tolerant and intolerant species. elevated sodium chloride had an effect on all species except for the tolerant longnose dace. meanwhile, lower dissolved oxygen and an increase in phosphorous affected both tolerant and intolerant species. overall, water contaminants pose a great threat to aquatic ecosystems as they can lower fecundity, decrease activity, impair growth and development and reduce survivability in fish