From WA DOT: "
Eco-regions denote areas within which ecosystems (the type, quality, and quantity of environmental conditions) are generally similar. The approach used to compile this map is based on the premise that ecological regions can be identified through the analysis of the patterns and the composition of biotic and abiotic phenomena that affect or reflect differences in ecosystem quality and integrity (Wiken 1986; Omernik 1987, 1995). These phenomena include geology, physiography, vegetation, climate, soils, land use, wildlife, and hydrology. The relative importance of each characteristic varies from one ecological region to another regardless of the hierarchical level. http://www.epa.gov/wed/pages/ecoregions/level_iii.htm"