A toxicity test can be performed one of two ways. One, ambient water samples are collected and bioassayed in a lab using a standard fish and/or a macroinvertebrate species. Other alternatives include the use of commercially available toxicity test systems that utilize microorganisms. Two, a mechanism to expose standard fish and/or macroinvertebrate species to ambient conditions in the waterway can be used. In either case, the result provides the gradation of toxicity spatially from upstream to downstream in the urban corridor. Both dry weather and wet weather samples could be taken. The result of this sampling provides a direct measure of toxicity, but it is still only a "snapshot" of the overall conditions in the receiving waters and does not contribute to an understanding of habitat constraints. For the purpose of identifying sources, samples from major outfalls and tributaries can be subjected to the same analysis.