Observed stream responses to changes in runoff quality

The question "What do we know about impacts of urbanization on receiving water?" can be easily answered. We know a lot, and we know almost nothing. The dual response is due to the fact that we have an abundance of studies that show change, but few studies that confirm cause and effect relationships between water quality and receiving system response. The identification of the relationships between water quality and the response of biological communities can begin with an assessment of potential toxicity associated with runoff events. Since toxicity is produced by a contaminant concentration and associated duration of exposure, assessing the toxicity of runoff should be straightforward. Unfortunately, there has been very little research supporting exposure regimes that are appropriate to storm events where peak concentrations of contaminants may last for only a few seconds. Further, toxicity is also associated with the frequency of exposure to a given concentration/duration of exposure couple. More frequent exposure events, at lower concentration may have more effect than less frequent exposure events at higher concentrations. What emerges from a careful review of this time-scale of exposure research on environmental toxicity research is that storm-related toxicity is not easily characterized. In short, we have lots of toxicity data, but little toxicity information to better understand urbanization effects on receiving systems. In addition to direct toxicity, bioassessments indicate that urbanization affects the integrity of receiving streams. Studies over the past twenty years have found aquatic communities indicative of degraded conditions in urban environments. Unfortunately, these studies fail to provide a specific community response that is specifically responsive to urban runoff. Again, we have lots of data, but information on the specific effects of urbanization is inconsistent. The following discussions will address the question of "What do we know?" by focusing more on what we should know! The following discussions will review a time-scale approach to information gathering that will provide a solid foundation for urban runoff effect analysis