Mapping watershed integrity for the conterminous United States.

Area of Science:

• Environmental Science
• Ecology
• Water Resource Management

Background:

• Watershed integrity is crucial for ecological sustainability.
• Existing datasets like EPA's StreamCat provide valuable information for assessment.
• Understanding watershed functions is key to managing water resources.

Purpose of the Study:

• To develop and map an Index of Watershed Integrity (IWI) for the conterminous US.
• To approximate relationships between environmental stressors and watershed functions.
• To evaluate the predictive power of the IWI and a related Index of Catchment Integrity (ICI) for ecological indicators.

Main Methods:

• Calculated and mapped the IWI using EPA's StreamCat data for 2.6 million watersheds.
• Approximated relationships between stressors and six key watershed functions.
• Employed regression analyses and multinomial logistic regression to assess IWI/ICI predictive capabilities against biological and water quality indicators.

Main Results:

• High watershed integrity was observed in the western US, with intermediate and low levels in the southern/eastern US and temperate plains/lower Mississippi Valley, respectively.
• The IWI showed significant, though sometimes modest, predictive power for water quality and biological indicators.
• The IWI and ICI were strong predictors of water quality, often outperforming land use variables.

Conclusions:

• The IWI provides a valuable tool for assessing watershed integrity at multiple scales.
• The index can be improved by incorporating more detailed stressor-function relationships.
• Despite limitations, the IWI offers a useful framework for guiding watershed management and conservation efforts.