Generally, wetlands are lands where saturation with water is the dominant factor determining the nature of soil development and the types of plant and animal communities living in the soil and on its surface (Cowardin, December 1979). Wetlands vary widely because of regional and local differences in soils, topography, climate, hydrology, water chemistry, vegetation, and other factors, including human disturbance. Indeed, wetlands are found from the tundra to the tropics and on every continent except Antarctica.
For regulatory purposes under the Clean Water Act, the term wetlands means "those areas that are inundated or saturated by surface or groundwater at a frequency and duration sufficient to support, and that under normal circumstances do support, a prevalence of vegetation typically adapted for life in saturated soil conditions. Wetlands generally include swamps, marshes, bogs and similar areas."
[taken from the EPA Regulations listed at 40 CFR 230.3(t)]
1987 Corps Wetland Delineation Manual and Regional Supplements: Manuals and supporting documents for identifying wetlands across the U.S. including peer reviews, public comments and draft regional supplements.
National Wetland Plant List: Botanical information on wetland plants from across the U.S. including biological attributes, distribution maps, pictures, references and more. Updated 2012 National Wetland Plant list published May 9, 2012.
Classification of Wetlands and Deepwater Habitats of the United States (Cowardin et al., 1979) (PDF) (142 pp, 17.03MB, About PDF) - Additional information on the characterization of wetlands and deep water habitats