From the site: "This dataset has two purposes: to provide users with a comprehensive set of geospatial characteristics for a large number of gaged watersheds, particularly for gages with long flow record; and to provide a determination of which of those watersheds represent hydrologic conditions which are least disturbed by human influences ("reference gages"), compared to other watersheds within 12 major ecoregions. Identifying reference gages serves important research goals: for example identifying conditions or goals for stream restoration, climate change studies, and more."