During the last four decades, several definitions of the term "wetland" have been offered by different sources. Today's legal and jurisdictional delineations were published in the Corps of Engineers Wetlands Delineation Manual and revised in 1989. It states that wetlands are "those areas that are inundated or saturated by surface or ground water at a frequency and duration sufficient to support, and that under usual circumstances support a prevalence of vegetation typically adapted for life in saturated soil conditions."
For an area to be a wetland, it must have certain hydrology , soils, and vegetation. Vegetation is dominated by species tolerant of saturated soil conditions. They exhibit a variety of adaptations that allow them to grow, compete, and reproduce in standing water or waterlogged soils lacking oxygen. Soils are wet or have developed under permanent or periodic saturation. The hydrologic cycle produces anaerobic soils, excluding a strictly upland plant community.
There are seven major types of wetlands that can be divided into two major groups: coastal and inland. Coastal wetlands are those that are influenced by the ebb and flow of tides and include tidal salt marshes, tidal freshwater marshes, and mangrove swamps. Salt marshes exist in protected coastlines in the middle to high latitudes. Plants and animals in these areas are adapted to salinity , periodic flooding , and extremes in temperature. These marshes are prevalent along the eastern and Gulf coasts of the United States as well as narrow belts on the west coast and along the Alaskan coastline.
Tidal freshwater marshes occur inland from the tidal salt marshes and host a variety of grasses and perennial broad-leaved plants. They are found primarily along the middle and south Atlantic coasts and along the coasts of Louisiana and Texas.
Mangrove swamps occur in subtropical and tropical regions of the world. Mangrove refers to the type of salt-tolerant trees that dominate the vegetation of this wetland. These wetlands are only found in a few places in the United States; the largest areas are found in the southern tip of Florida.
Inland wetlands, which constitute the majority of wetlands in the United States, occur across a variety of climatic zones. They can be divided into four types: northern peat-lands , southern deep water swamps, freshwater marshes, and riparian ecosystems.
Freshwater marshes represent a variety of different inland wetlands. They have shallow water, peat accumulation, and grow cattails, arrowheads, and different species of grasses and sedges. Major freshwater marshes include the Florida Everglades , Great Lakes coastal marshes, and areas of Minnesota and the Dakotas.
Southern deepwater swamps are freshwater woody wetlands with standing water for most of the growing season. The most recognizable type of vegetation is cypress. They are either fed by rainwater or occur in alluvial positions that are annually flooded. These wetlands are found in the southeast United States.
Northern peatlands consist of deep accumulation of peat. Primary locations are Minnesota, Wisconsin, Michigan, areas of the Northeast that have been affected by the last glaciation , and some mountain and coastal bays in the southeast. Bogs are marshes or swamps that lack contact with local groundwater and are acidified by organic acids from plants. They are noted for nutrient deficiency and waterlogged conditions with vegetation adapted to conserve nutrients in this environment .
Riparian forested wetlands, occurring along rivers and streams, are occasionally flooded but generally dry for a large part of the growing season. The most common type of wetland in the United States, they are often productive because of the periodic addition of nutrients with sediment deposited during floods.
Wetlands are valuable in several ways. Because of their appearance and biodiversity alone, wetlands are a valuable resource. Many types of wildlife , including endangered species such as the whooping crane and the alligator, inhabit or use wetlands. Over 50% of the 800 species of protected migratory birds rely on wetlands. Wetlands are valuable for recreation , attracting hunters of ducks and geese. Over 95% of the fish and shellfish that are taken commercially depend on wetland habitat in their life cycles.
Forest wetlands are an important source of lumber. Other wetland vegetation, such as cattails or woody shrubs, could someday be harvested for energy production. Peat is used in potted plants and as a soil amendment, particularly to grow grass sod.
Wetlands intercept and store storm waters, reducing the peak runoff and slowing stream discharges, reducing flood damage. In coastal areas, wetlands act as buffers to reduce the energy of ocean storms before they reach more populated areas and cause severe damage. Although most wetlands do not, some may recharge underlying ground-water. Wetlands can improve surface water quality by the removal of nutrients and toxic materials as water runs over or through it. Most importantly, wetlands may play a significant role in the global cycling of nitrogen , sulfur, methane , and carbon dioxide .
The current movement of conservation has encouraged the "reconstruction" of wetlands that have been destroyed through a no-net-loss policy. Wetlands are restored to protect coastlines, improve water quality, and replace lost habitat.
[James L. Anderson ]
Kusler, J. A., and M. E. Kentula, eds. Wetland Creation and Restoration: The Status of the Science. Covelo, CA: Island Press, 1990.
Mitsch, W. J., and J. G. Gorselink. Wetlands. New York: Van Norstrand Reinhold, 1993.
Williams, M., ed. Wetlands: A Threatened Landscape. Cambridge, MA: Basil Blackwell, 1990.
A Citizen's Guide to Protecting Wetlands. Washington, DC: National Wildlife Federation, 1989.
IUCN Environmental Law Centre staff, eds. The Legal Aspects of the Protection of Wetlands. Gland, Switzerland: IUCN—The World Conservation Union, 1989.