Griffith, Glenn, James Omernik, and Alan Woods. 1999. Ecoregions, watersheds, basins, and HUCís: how state and federal agencies frame water quality. Journal of Soil and Water Conservation 54(4):666-677.
Many state and federal agencies have adopted a "watershed approach" for water quality assessment and management, and the U.S. EPA recommends using hydrologic units for this purpose. Watersheds are important spatial units for studies of land-water relationships, but most hydrologic units are not watersheds. More importantly, watersheds, basins, or hydrologic units do not correspond to the spatial patterns of regional characteristics such as physiography, soils, vegetation, geology, climate, and land use that influence the physical, chemical, or biological nature of water bodies. For effective management strategies regarding protective water quality standards or restoration goals, these regional differences in ecological potentials should be considered. An ecoregion framework is an appropriate and necessary complementary tool for watershed assessment and management. Reference watersheds within ecoregions can be used to help set expectations, standards, and management practices. National, regional, and state examples illustrate the need to recognize the limitations of water quality assessments conducted solely within watershed or hydrologic unit frameworks.