Percent impervious is the percentage of the land area within a watershed covered with impervious
surfaces, or surfaces that do not allow water to infiltrate the soil. They include rooftops and asphalt or concrete
roads, parking lots and sidewalks. Find out more about how this data was analyzed
here.
The Stream Impact Threshold is defined by the Nonpoint Education for Municipal Officials and is cited in "Center for Watershed
Protection, 8391 Main Street, Ellicott City, MD, 21043-4605.
More information on how these numbers were calculated can be found on the
impervious surface page of this website.
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