Risk-based assessments of anthropogenic impacts on freshwaters commonly integrate conceptual models, expert knowledge, and impact-related data to prioritize watersheds for conservation efforts.  We developed a new risk-based tool, the Ecological Risk Index (ERI), which uses a widely accepted conceptual model of biological integrity to integrate remotely-sensed land/water use data with expert knowledge of environmental impacts, and then ranks selected watersheds in terms of the extent and severity of threats to freshwater biota.  The risk-based planning approach can incorporate a wide range of data quality and accommodate short timelines, but has been criticized for its lack of objectivity and statistical rigor.

To evaluate the ERI’s reliability, we analyzed its sensitivity to the subjectivity of, and differences among, experts’ judgment. We found that experts produced conservative estimates of ecological risk and added knowledge crucial to accurately quantifying threats to biota. We addressed uncertainties related to risk categorization by applying a multivariate approach to class delineation.  Unfortunately, relevant biological data were often absent when assigning degradation thresholds.  We have mapped threats for the upper Tennessee River basin and throughout South America, and found that the ERI accommodates various threat types and is applicable to conservation planning across a wide range of spatial scales.