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What is a Wetland?
Although federal agencies, states, and text book authors vary in the way in which they define wetlands, in general terms, wetlands are lands on which water covers the soil or is present either at or near the surface of the soil or within the root zone, all year or for varying periods of time during the year, including during the growing season. The recurrent or prolonged presence of water (hydrology) at or near the soil surface 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. Wetlands can be identified by the presence of those plants (hydrophytes) that are adapted to life in the soils that form under flooded or saturated conditions (hydric soils) characteristic of wetlands (NAS 1995; Mitsch and Gosselink 1993). There also are wetlands that lack hydric soils and hyrdrophytic vegetation, but support other organisms indicative of recurrent saturation (NAS 1995).
The federal regulations implementing Section 404 of the Clean Water Act define wetlands as:
Those areas that are inundated or saturated by surface or ground water (hydrology) at a frequency and duration sufficient to support, and that under normal circumstances do support, a prevalence of vegetation (hydrophytes) typically adapted for life in saturated soil conditions (hydric soils). Wetlands generally include swamps, marshes, bogs, and similar areas (40 CFR 232.2(r)).

Jurisdictional wetlands -- those that are regulated by the U.S. Army Corps of Engineers (Corps) under Section 404 -- must exhibit all three haracteristics: hydrology, hydrophytes, and hydric soils (US ACOE 1987). It is important to understand that some areas that function as wetlands ecologically, but exhibit only one or two of the three characteristics, do not currently qualify as Corps jurisdictional wetlands and thus activities in these wetlands are not regulated under the Section 404 program. Such wetlands, however, may perform valuable functions.

Another federal agency, the U.S. Fish and Wildlife Service defines wetlands as: lands that are transitional between terrestrial and aquatic systems where the water table is usually at or near the surface or the land is covered by shallow water, and that have one or more of the following attributes:

At least periodically, the land supports predominantly hydrophytes; the substrate is predominantly undrained hydric soil; and, the substrate is non-soil and is saturated with water or covered by shallow water at some time during the growing season of each year (Cowardin et al. 1979).

This definition differs from the EPA and U.S. Army Corps of Engineers definition used for jurisdictional wetlands which requires that all three attributes (hydrophytes, hydric soils, and hydrology) be evident. The 1987 Corps of Engineers Manual on wetland delineation does not consider unvegetated aquatic sites such as mudflats and coral reefs or vegetated shallow water to be wetland areas, whereas the Cowardin classification does (US ACOE 1987).