Wetlands are transition zones between land and aquatic systems where the water table is usually near or at the surface, or the land is covered by shallow water. Wetlands can take many forms, some of which are immediately recognizable as "wet." Other wetlands appear more like dry land, and are wet during only certain seasons of the year, or at several year intervals. In fact, the U.S. Army Corps of Engineers reports that most wetlands lack surface water and waterlogged soils during at least part of each growing season.
Swamps are wetlands that are dominated by trees and shrubs. Swamp forests that are associated with rivers and streams in the Southeast are commonly known as bottom-land hardwoods. Wetlands that consist of herbaceous plants, such as sedges, cattails, and bulrushes, are known as marshes. Marshes are highly variable and include fens, sloughs, potholes, and wet meadows. Bogs are generally dominated by sphagnum moss, which builds thick layers of peat as it dies. Other wetlands include seeps, vernal pools, pocosins, and muskegs. Although many people do not think of them as such, the deep channels of rivers, streambeds, lake bottoms, and shallow tidal waters are also wetlands.
According to the Environmental Protection Agency (EPA) in "Wetlands: Status and Trends" (http://www.epa.gov/OWOW/wetlands/vital/status.html, March 23, 2005), as of 1997 there were an estimated 105.5 million acres of wetlands in the forty-eight conterminous states (does not include Alaska and Hawaii), which is about 5.5% of the total land area. Ninety-five percent of these wetlands were freshwater wetlands, while 5% were estuarine (coastal saltwater) wetlands. Wetlands range in size from less than one acre to thousands of acres.
In "Wetlands: Status and Trends," the EPA reported that in the 1980s an estimated 170 to 200 million acres of wetlands existed in Alaska—covering slightly more than half of the state—while Hawaii had 52,000 acres. After Alaska, Florida (eleven million acres), Louisiana (8.8 million), Minnesota (8.7 million), and Texas (7.6 million) have the largest wetland acreage.
Indispensable Part of Life on Earth
Wetlands are distributed unevenly, but occur in every state and U.S. territory. (See Table 7.1.) They are found wherever climate and landscape cause groundwater to discharge to the land surface or prevent rapid drainage from the land surface so that soils are saturated for some time. All wetlands have one common trait: hydric (oxygen-poor) soils. Wetlands are covered by shallow water or have water just below the ground surface long enough to create waterlogged soils for long periods during the growing season. These conditions cause hydric soils.
Almost all plants and animals use oxygen to convert sugar and other organic molecules into the energy that they need to grow and survive. When soil microbes decompose dead plants and animals, they use oxygen that is trapped in the soil. Normally that oxygen is replaced from the air.
In wetlands, when the hydric soil is flooded or saturated, the oxygen used by the microbes is not replaced fast enough. This is because oxygen moves through water about 10,000 times slower than through air. As a result, wetland plants are specially adapted to temporarily survive without oxygen in their roots or to transfer oxygen from the leaves or stem to the roots. This anaerobic (without oxygen) condition also causes the soils to have the sulfurous odor of rotten eggs.
Local hydrology (the pattern of water flow through an area) is the primary determinant of wetlands. Wetlands can receive groundwater in-flow, recharge groundwater, or experience both inflow and outflow at different locations. Figure 7.1 illustrates water movement in several different wetland situations. Wetlands do not
Location of various wetland types
SOURCE: "Table 3. Locations of Various Wetland Types in the Unites States," in Wetlands: Their Use and Regulation, U.S. Congress, Office of Technology Assessment, 1984
|Inland freshwater marsh
||Dakota-Minnesota drift and lake bed; Upper Midwest; and Gulf Coastal Flats
||North Dakota, South Dakota, Nebraska, Minnesota, Florida
|Inland saline marshes
||Intermontane; Pacific Mountains
||Oregon, Nevada, Utah, California
||Upper Midwest; Gulf-Atlantic Rolling Plain; Gulf Coastal Flat; Atlantic Coastal Flats
||Wisconsin, Minnesota, Michigan, Maine, Florida, North Carolina
||Central Highland and Basin; Arctic Lowland; and Pacific Mountains
||Upper Midwest; Gulf Coastal Flats
||Minnesota, Wisconsin, Michigan, Florida, Georgia, South Carolina, North Carolina, Louisiana
||Upper Midwest; Gulf Coastal Flats; Atlantic Coastal Flats; and Lower Mississippi Alluvial Plain
||Minnesota, Wisconsin, Michigan, Florida, Georgia, South Georgia, South Carolina, North Carolina, Louisiana
|Bottom land hardwood
||Lower Mississippi Alluvial Plain; Atlantic Coastal Flats; Gulf-Atlantic Rolling Plain; and Gulf Coastal Flats
||Louisiana, Mississippi, Arkansas, Missouri, Tennessee, Alabama, Florida, Georgia, South Carolina, North Carolina, Texas
|Coastal salt marshes
||Atlantic Coastal Zone; Gulf Coastal Zone; Eastern Highlands; Pacific Moutains
||All Coastal States, but particularly the Mid- and South Atlantic and Gulf Coast States
||Gulf Coastal Zone
||Florida and Louisiana
|Tidal freshwater wetlands
||Atlantic Coastal Zone and Flats; Gulf Coastal Zone and Flats
||Louisiana, Texas, North Carolina, Virginia, Maryland, Delaware, New Jersey, Georgia, South Carolina
Examples of water sources for wetlands
SOURCE: Thomas C. Winter, Judson W. Harvey, O. Lehn Franke, and William M. Alley, "Figure 17," in Ground Waters and Surface Waters—A Single Resource, United States Geological Survey, 1998
always occupy low points and depressions in the landscape. They can occur at the soil interface with complex underground water systems. (See Figure 7.1, part A.) Fens are examples of wetlands that occur on slopes at groundwater seepage faces and are subject to a continuous supply of the chemicals that are dissolved in the groundwater. (See Figure 7.1, part B.) Locations that are down gradient of a break in the slope of the water table, such as along streams or rivers, receive a continuous water supply and are ideal for wetland growth. They also, however, may receive some groundwater discharge. (See Figure 7.1, part C.) Bogs are wetlands normally found on uplands or extensive flatlands. Most of their water and chemistry comes from precipitation. (See Figure 7.1, part D.)
Riverine (areas along streams, rivers, and irrigation canals) and coastal area wetlands are highly subject to periodic water level changes. Coastal area wetlands, for example, are affected by predictable tidal cycles. Other coastal and riverine wetlands are highly dependent on flooding and seasonal water level changes. Some examples are the floodplains of the Illinois and Missouri Rivers.