In 1979, a comprehensive classification system of wetlands and deepwater habitats was developed for the U.S. Fish and Wildlife Service (Cowardin et al. 1979). Under this system, wetlands are of two basic types: coastal (also known as tidal or estuarine wetlands) and inland (also known as non-tidal, freshwater, or palustrine wetlands).
Coastal wetlands are found along the Atlantic, Pacific, Alaskan, and Gulf coasts and include estuaries. The salt water and tides combine to create an environment in which most plants, except salt-tolerant species (halophytes), cannot survive. Mangrove swamps, dominated by halophytic shrubs or trees, are common in warm climates, for example, in southern Florida, Puerto Rico, and Louisiana. Tidal freshwater wetlands form in upstream coastal wetlands where the influence of salt water ends.
Inland wetlands include floodplains along rivers and streams (e.g., bottomlands and other riparian wetlands); isolated depressions surrounded by dry land (e.g., prairie potholes); areas where the groundwater intercepts the soil surface (e.g., fens) or where precipitation saturates the soil for a season or longer (e.g., vernal pools and bogs). Marshes and wet meadows are dominated by grasses and other herbaceous plants or shrubs; and swamps are dominated by trees. Certain types of inland wetlands are common to particular regions of the country: the Carolina bays and pocosins in the Southeast; bogs and fens in the northeastern and north-central states and Alaska; inland saline and alkaline marshes, playas, and riparian wetlands in the arid and semiarid West; prairie potholes, including the Nebraska Sandhills, in the northern Midwest; and bottomland hardwood swamps of the South.
The USFWS's Cowardin classification system defines deepwater habitats are defined as: permanently flooded lands lying below the deepwater boundary of wetlands (2 meters), including environments where surface water is permanent, with water, rather than air, the principal medium within which the dominant organisms live.
The Cowardin system is hierarchical and includes several layers of detail for wetland classification including: a subsystem of water flow; classes of substrate types; subclasses of vegetation types and dominant species; as well as flooding regimes and s alinity levels for each system. This system is appropriate for an ecologically based understanding of wetland definition. The entire Classification of Wetlands and Deepwater Habitats of the United States, including tables and figures of the hierarchical structure, is available online.
Marine - Open ocean overlying the continental shelf and coastline exposed to waves and currents of the open ocean shoreward to (1) extreme high water of spring tides; (2) seaward limit of wetland emergents, trees, or shrubs; or (3) the seawa rd limit of the Estuarine System, other than vegetation. Salinities exceed 30 parts per thousand.
Estuarine - Deepwater tidal habitats and adjacent tidal wetlands that are usually semi-enclosed by land but have open, partly obstructed, or sporadic access to the ocean, with ocean water at least occasionally diluted by freshwater runoff from the land. The upstream and landward limit is where ocean-de rived salts measure less than .5 parts per thousand during the period of average annual low flow. The seaward limit is (1) an imaginary line closing the mouth of a river, bay, or sound; and (2) the seaward limit of wetland emergents, shrubs, or trees whe n not included in (1).
Riverine - All wetlands and deepwater habitats contained within a channel except those wetlands (1) dominated by trees, shrubs, persistent emergents, emergent mosses, or lichens, and (2)which have habitats with ocean-derived salinities in excess of .5 parts per thousand.
Lacustrine - Wetlands and deepwater habitats (1) situated in a topographic depression or dammed river channel; (2) lacking trees, shrubs, persistent emergents, emergent mosses, or lichens with greater than 30% areal coverage; and (3)whose total area exceeds 8 hectares (20 acres); or area less than 8 hectares if the boundary is active wave-formed or bedrock or if water depth in the deepest part of the basin exceeds 2 m (6.6 ft) at low water. Ocean-derived salinities are always less than .5 parts per thousand.
Palustrine - All nontidal wetlands dominated by trees, shrubs, persistent emergents, emergent mosses, or lichens, and all such tidal wetlands where ocean-derived salinities are below .5 ppt. This category also includes wetlands lacking suc h vegetation but with all of the following characteristics: (1) area less than 8 ha; (2) lacking an active wave-formed or bedrock boundary; (3) water depth in the deepest part of the basin less than 2 m (6.6 ft) at low water; and (4) ocean-derived salinit ies less than .5 parts per thousand.
A Palustrine system can exist directly adjacent to or within the Lacustrine, Riverine, or Estuarine systems.