Marshes, swamps, ponds, and bogs are teeming biological nurseries for migratory birds, fish, and aquatic plants. They also provide natural flood and erosion control. These
predominantly wet area, or wetlands as they are commonly called, now represent only about 5 percent of the land surface of the lower 48 states. Of 215 million acres of wetlands that one existed in the conterminous United States, the U.S. Fish and Wildlife Service (FWS) estimates that about 99 million
acres are left.
Each year, development, drainage, and agriculture eliminate another 458,000 acres - an area more than half the size of Rhode Island. Conversions of wetland to farmland during a recent 20-year period caused 87 percent of all wetland losses.
The FWS has long recognized the importance of America's wetlands because they form the breeding and wintering grounds for great numbers of migratory birds. In 1977, the FWS began the National Wetlands Inventory (NWI), a systematic effort to classify and map America's remaining wetlands.
According to this system wetlands are defined as lands 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. For purposes of this classification, wetlands must have one or more of the following three attributes:
Since plants and soils furnish a record of the hydrology of a site, they form the basis of the hierarchical classification scheme that divides wetlands into five major systems: marine, estuarine, riverine, lacustrine, and palustrine.
Working with the classification guide and
Classification Scheme
The NWI describes wetlands according to the "Classification of Wetlands and Deepwater Habitats of the United States", a system that describes wetlands by soils, hydrology, and vegetation.
Map coverage
Wetland maps exist for about 75 percent of the conterminous United States, 24 percent of Alaska, and all of Hawaii. The wetlands given the highest mapping priority are portions of the coastal zone; flood plains of major rivers; the Midwest "prairie pothole" region, an area that is a major breeding ground for ducks; and the lower Mississippi alluvial plain. To date, work in 16 States has been completed and work continues in 34 others.Applications
More than 32,000 composite and overlay maps produced to date by the NWI are used in a variety of ways, including land-use planning and guides, town planning, wildlife habitat
management, water-quality planning, project studies, soil and water conservation loans, zoning, flood hazard planning, research, waste treatment, permit reviews, and flood controls.Map specifications
Two types of maps are produced by the NWI: composite maps that photographically combine the wetlands inventory information with standard U.S. Geological Survey (USGS) topographic map information, and overlays to these maps that contain wetland information only. Both types are available on a paper base that resembles a blueprint in appearance or as transparencies on frosted
Mylar film. The wetlands inventory transparencies can be laid on top of USGS 7.5-minute, 15-minute, and 30- by 60-minute topographic maps that served as base maps for the overlays. Both the composites and overlays display the location, shape, and characteristics of wetlands.