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 …