The Environmental Protection Agency (EPA) and the U.S. Army Corps of Engineers (USACE) jointly define wetlands as: Those areas that are inundated or saturated by surface or ground water 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.

Wetlands have three diagnostic environmental characteristics:

1. Vegetation: Vegetation that is adapted to growing in water or in soils periodically deficient in oxygen as a result of excessive water content.
2. Soils: Hydric soils that formed under conditions of saturation, flooding, or ponding long enough during the growing season to develop anaerobic conditions.
3. Hydrology: Hydrology causing the soil to be saturated to the surface at some time during the growing season.

Under CWA, a positive wetland indicator from each characteristic must be found in order for an area to be determined a wetland. This definition is more restrictive than the classification system used for the National Wetlands Inventory, in which a site may be identified as a wetland based solely on the hydrology of the site. 

USACE is the authoritative source for determining wetlands under CWA. Not all wetlands identified in the USFWS National Wetlands Inventory classify as a wetland under CWA.