Citation:

Thornbrugh, D., S. Leibowitz, R. Hill, M. Weber, Z. Johnson, Tony Olsen, J. Flotemersch, J. Stoddard, AND D. Peck. Mapping watershed integrity for the conterminous United States.. ECOLOGICAL INDICATORS. Elsevier Science Ltd, New York, NY, 85:1133-1148, (2018).

Impact/Purpose:

As part of the Clean Water Act, the EPA is required to report on the integrity of the Nation’s water resources. Under the Safe and Sustainable Water Resources National Program, work is being conducted to evaluate, assess, and improve an approach for national mapping of watershed integrity. Watershed integrity is the capacity of a watershed to support and maintain the full range of ecological processes and functions essential to sustainability. Here we evaluated and mapped an Index of Watershed Integrity (IWI) for 2.6 million watersheds in the conterminous US using EPA’s StreamCat dataset. Results show high integrity in the western US, intermediate integrity in the southern and northeastern US, and the lowest integrity in the upper midwest and lower Mississippi Valley. A related Index of Catchment Integrity (ICI) was developed using local drainages of individual stream segments (i.e., excluding upstream information). Although related to the IWI, the ICI could be useful for local applications where information on a mainstem river and its upstream catchments is not required. The IWI was able to account for a quarter of the national variation in a water quality metric that was derived using data from EPA’s National Rivers and Streams Assessment. The IWI in its present form could be useful for management efforts at multiple scales, especially when combined with information on site condition. For example, this information could help target restoration at poor condition sites that occur within high integrity watersheds. However, limitations concerning the absence of data for certain stressors should be considered. In particular, a source of data for cattle density is not available nationally. This means that integrity is overestimated in areas, such as the southwest, where grazing is important. This work is one of three manuscripts contributing to the FY16 SSWR Annual Performance Reporting (APR) product 3.01B.1, “National maps of watershed integrity and stream condition and report and webinar describing these.” The results of this study could be important for three programs within the Office of Water: the National Aquatic Resource Surveys, the Healthy Watersheds Program, and the Biocriteria Program.

Description:

Watershed integrity is the capacity of a watershed to support and maintain the full range of ecological processes and functions essential to sustainability. We evaluated and mapped an Index of Watershed Integrity (IWI) for 2.6 million watersheds in the conterminous US using first order approximations of relationships between stressors and six watershed functions, along with information from EPA’s StreamCat dataset. Results show high integrity in the western US, intermediate integrity in the southern and northeastern US, and the lowest integrity in the upper midwest and lower Mississippi Valley. Correlation between the six functional components (hydrologic regulation, regulation of water chemistry, sediment regulation, hydrologic connectivity, temperature regulation, and habitat provision) was high (Pearson correlation coefficient of 0.86-0.98), but this correlation could be reduced and the IWI improved by including actual functional relationships between components and stressors. A related Index of Catchment Integrity (ICI) was developed using local drainages of individual stream segments (i.e., excluding upstream information). Although correlated with the IWI (Pearson correlation coefficient = 0.85), the ICI could be useful when information on a mainstem river and its upstream catchments is unnecessary. We utilized regression analyses to evaluate the ability of the IWI and ICI to predict six site-level indicators – three biological indicators and principal components derived from water quality, habitat, and combined water quality and habitat variables – using data from EPA’s National Rivers and Streams Assessment. Relationships were highly significant, but the IWI only accounted for 2-12% of the variation in the four biological and habitat variables. The IWI accounted for a quarter of the variation in the water quality and combined principal components nationally, and over 30% in the Northern and Southern Appalachians. Better predictive ability should be possible by incorporating actual relationships between functional components and stressors. The IWI in its present form could be useful for management efforts at multiple scales, especially when combined with information on site condition. However, limitations concerning the absence of data for certain stressors should be considered.

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