Causal Assessment in a Complex Urban Watershed—The Long Creek Case Study *

The Long Creek case study illustrates the application of causal assessment in an urban watershed with many sources and stressors. The case highlights the comparison of conditions in Long Creek to those in a nearby but less affected stream, and to stressor–response relationships from other field and laboratory studies. Specific effects may be of greater benefit to causal assessment than percentages, proportions, and indexes. The more complex the interactions between biological response and candidate cause, the more difficult it is to identify mechanisms by which changes occur and to draw conclusions. The results of the causal assessment are being used to inform ongoing watershed-level activity, including discharge permitting and evidence-based sustainability efforts.