Aquatic ecoregions of the conterminous United States

Ecoregions are based on perceived patterns of a combination of causal
and integrative factors including land use, land surface form, potential
natural vegetation, and soils (Omernik, 1987). This is a copy of the
ecoregion coverage of Omernik (1987) with some item names modified.
This is NOT the most recent version of the ecoregion definitions,
which can be obtained from: Environmental Protection Agency, Western
Ecology Division, Corvallis, OR at
http://www.epa.gov/wed/pages/ecoregions/level_iii.htm.