USGS Water Resources
National Water Summary on Wetland Resources
United States Geological Survey Water Supply Paper 2425

Wetland Management and Research
Wetland Mapping and Inventory

By Bill O. Wilen, U.S. Fish and Wildlife Service
Virginia Carter, U.S. Geological Survey, and
J. Ronald Jones, U.S. Geological Survey

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Wetland maps are a prerequisite for wetland inventory and for wetland development planning, management, protection, and restoration. Maps provide information on wetland type, location, and size. Detailed wetland maps are necessary for analysis of the effect of projects at specific sites and for providing baseline spatial data for the assessment of the effects of national policies and activities. Wetland maps are used by local, State, and Federal agencies, as well as by private industry and organizations. They are used for many purposes, including the development of comprehensive resource management plans, environmental impact assessments, natural resource inventories, habitat surveys, and the analysis of trends in wetland status.

Several Federal agencies map wetlands in support of their Congressional mandate. These include the U.S. Department of the Interior, U.S. Fish and Wildlife Service (FWS); the U.S. Department of Agriculture, Natural Resources Conservation Service (NRCS); and the U.S. Department of Commerce, National Oceanic and Atmospheric Administration (NOAA). The FWS has the primary responsibility for mapping and inventory of all the wetlands of the United States. The wetland maps produced by other agencies serve different purposes and generally involve cooperation with the FWS.


The FWS National Wetlands Inventory is responsible for the mapping and inventory of wetlands throughout the United States. The Emergency Wetlands Resources Act of 1986 and amendments to it in 1988 and 1992 define the responsibilities of the National Wetlands Inventory. (See the article "Wetland Protection Legislation" in this volume for more information on this and other wetland legislation.)

Despite all the government legislation, policies, and programs, wetlands will not be protected if the regulations are not enforced. Perhaps the best way to protect wetlands is to educate the public of their benefits. If the public does not recognize the benefits of wetland preservation, wetlands will not be preserved. Protection can be accomplished only through the cooperative efforts of citizens.

Wetland maps are a prerequisite for wetland inventory, planning, management, protection, and restoration.

History and Status of the National Wetlands Inventory

In 1906, and again in 1922, the U.S. Department of Agriculture inventoried the wetlands of the United States to identify those that could be drained and converted to other uses (Wilen and Tiner, 1993). In 1954, the first nationwide wetland survey by the FWS covered about 40 percent of the conterminous United States and focused on important waterfowl wetlands. This survey was not comprehensive by today's standards, but it stimulated public interest in the conservation of waterfowl wetlands (Shaw and Fredine, 1956). (See the article "Wetlands as Bird Habitat" in this volume.)

After the earlier inventories, and in response to passage of the Emergency Wetlands Resources Act and its amendments, the FWS established the National Wetlands Inventory. The program is designed to (1) produce detailed maps on the characteristics and extent of the Nation's wetlands, (2) construct a national wetlands data base, (3) disseminate wetland maps and digital data, (4) report results of State wetland inventories, (5) report to Congress every 10 years on the status and trends of the Nation's wetlands, and (6) assemble and distribute related maps, digital data, and reports.

The National Wetlands Inventory has produced more than 50,800 maps covering 88 percent of the conterminous United States, 30 percent of Alaska, and all of Hawaii and the U.S. Territories (fig. 44) Priorities for mapping have been based on the needs of the FWS, other Federal agencies, and State agencies. To date, mapping has been concentrated on the coastal zone (including the Great Lakes), prairie wetlands, playa lakes, flood plains of major rivers, and areas that reflect goals of the North American Waterfowl Management Plan (U.S. Fish and Wildlife Service, 1976). As a practical matter, priorities have been based on the availability of funding and the availability of high-quality aerial photographs. The National Wetlands Inventory produced maps at a rate of about 5 percent of the conterminous United States and about 2 percent of Alaska annually through 1995-about 3,200 1:24,000-scale maps in the conterminous United States and about 60 1:63,360-scale maps in Alaska.

The Clean Water Act regulates dredge and fill activities that would adversely affect wetlands.
Figure 44

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Figure 44. Areas of the United States that have been mapped by the National Wetlands Inventory program and status of those maps, 1996. (Source: Data from U.S. Fish and Wildlife Service, National Wetlands Inventory files.)
The National Wetlands Inventory has published a series of documents on the trends in wetland losses and gains. The first of these reports was "Status and Trends of Wetlands and Deepwater Habitats in the Conterminous United States, 1950's to 1970's" (Frayer and others, 1983). In the Emergency Wetlands Resources Act of 1986 and subsequent amendments, Congress directed the National Wetlands Inventory to (1) update and improve the information contained in this report by 1990 and at 10-year intervals thereafter and (2) estimate the number of acres of wetland habitat in each State in the 1780's and the 1980's and calculate the percentage of loss in each State. In response to this directive, the National Wetlands Inventory published a 1990 report to Congress titled "Wetlands-Losses in the United States, 1780's to 1980's" (Dahl, 1990).

The National Wetlands Inventory also is preparing a geographically referenced digital data base for wetlands so that wetland information can be placed in geographic information systems (GIS) for use with computers. These digital maps and information are easily transmitted over the Internet. To date, almost 18,800 maps, representing 29 percent of the United States, have been digitized (fig. 45). Statewide data bases have been digitized for Delaware, Hawaii, Indiana, Maryland, Illinois, New Jersey, Washington, Iowa, Minnesota, and West Virginia. Digitization is in progress for Florida, North Carolina, South Carolina, South Dakota, and Virginia. Wetland digital data are available for parts of 35 other States.

In addition to wetland maps and status and trend reports, the National Wetlands Inventory produces special items related to the identification, mapping, and inventory of wetlands. The "National List of Plant Species that Occur in Wetlands" (Reed, 1988) is an important tool for identifying wetlands on the basis of their vegetation. A computerized data base for wetland plants, developed by the National Wetlands Inventory, also lists plants found in wetlands and ranks their affinity to the wetland environment. This information is important for determining whether an area is really a wetland. Additionally, the National Wetlands Inventory has contributed to a list of hydric soils (soils found in wetlands) (U.S. Soil Conservation Service, 1991). Many published State wetland reports, including "Wetlands of Maryland" (Tiner and Burke, 1995), "Wetlands of Connecticut" (Metzler and Tiner, 1992), and "Status of Alaska Wetlands" (Hall, Frayer, and Wilen, 1994), contain wetland inventory results and other important information. Finally, in cooperation with the U.S. Geological Survey (USGS), the National Wetlands Inventory has published a map (scale of 1 inch equals 50 miles) showing the locations of major wetland complexes in the conterminous United States, Hawaii, and Puerto Rico (Dahl, 1991) and a map (scale of 1 inch equals 40 miles) of Alaska's wetland resources (Hall, 1991).

To date, almost 18,800 maps, representing 29 percent of the United States, have been digitized.
Figure 45

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Figure 45. Areas of the conterminous United States and Hawaii where wetland data have been digitized by the National Wetlands Inventory program, 1996. (Source: Data from U.S. Fish and Wildlife Service, National Wetlands Inventory files.)


Natural Resources Conservation Service.--The NRCS (formerly the Soil Conservation Service) conducts its wetland inventory under the auspices of the wetland conservation provision (nicknamed "Swampbuster") of the Food Security Act of 1985. This Act provides for the reduction of a farmer's program benefits if wetlands are converted to agricultural production. In order to implement this act, the mapping of the NRCS is focused on freshwater wetlands that have a high potential for agricultural conversion, such as those adjacent to or lying within the boundaries of existing agricultural fields.

The NRCS does not produce a standard map product. Many delineations are made on l:660-scale black-and-white aerial photographs; others are made on soil-survey base maps at scales that range from 1:10,000 to 1:64,000 (Teels, 1990). Information sources for this program include recent and historical aerial photographs, such as those regularly acquired by the U.S. Department of Agriculture, National Wetlands Inventory maps from the FWS, U.S. Department of Agriculture crop history records, and field verifications.


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The "Swampbuster" discourages the conversion of wetlands to cropland. This wetland, which was converted to cropland at one time, has been restored. (Photograph courtesy of the U.S. Fish and Wildlife Service.)

National Oceanic and Atmospheric Administration.--The NOAA has developed the Coastal Wetland Habitat Change Program in order to delineate coastal wetland habitats and adjacent uplands and plains to monitor changes in these habitats on a cycle of 1 to 5 years. The basis for monitoring will be a data base describing the areal extent and distribution of coastal wetlands in the conterminous United States. The program will help to determine the linkages between estuarine and marine wetlands, as well as the distribution, abundance, and health of living marine resources.


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The National Oceanic and Atmospheric Administration delineates coastal weland and upland habitats, such as this coastal wetland at Chincoteague National Wildlife Refuge on Assateague Island, Va. (Photograph by Judy D. Fretwell, U.S. Geological Survey.)

U.S. Geological Survey.--The USGS compiles, produces, and disseminates topographic, hydrologic, and geolog-c maps and digital data related to wetlands. The standard USGS 1:24,000-scale topographic map commonly is used as a base for wetland mapping by other Federal, State, and local agencies. However, because USGS maps depict wetlands as unbounded symbols (fig. 46), the maps cannot be used to establish exact boundaries for wetlands. Intermediate-scale (1:100,000) and large-scale maps (scales of 1:24,000 or greater) are used for project planning. Large-scale maps known as orthophoto quadrangles, which are made by manipulation of aerial photographs to achieve a positionally accurate photographic base map, are used as a base for State wetland mapping.

Figure 46

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Figure 46. Unbounded symbols on a U.S. Geological Survey topographic map show the general location of wetlands.



Differing needs of various Federal agencies can require different types of maps or different map scales. However, many needs can be satisfied by common products, and efforts are being made to standardize maps and map products whenever possible or practical. Federal digital wetland mapping is coordinated by the Wetlands Subcommittee of the Federal Geographic Data Coordination group in an effort to meet requirements established by the Office of Management and Budget. The Office of Management and Budget requires agencies to develop a national digital spatial information resource in collaboration with State and local governments and the private sector. This requirement is for the purposes of (1) promoting the development, maintenance, and management of a national digital wetland data base; (2) encouraging the development and implementation of standards, exchange formats, specifications, procedures, and guidelines; (3) promoting interaction among other Federal, State, and local government agencies that have interests in the generation, collection, use, and transfer of wetland spatial data; (4) maintaining and disseminating information on the type and availability of wetland spatial data; and (5) promoting the concept of effective wetland management.

"Swampbuster" removes Federal incentives for the agricultural conversion of wetlands.


Most natural-resource inventories make use of aerial photographs or satellite images combined with field verification. The National Wetlands Inventory uses the best and most appropriate aerial photographs available for mapping wetlands. The principal data source in the early 1980's was the 1:80,000-scale, high-altitude, black-and-white aerial photography acquired by the USGS for topographic mapping and production of orthophoto quadrangles. After the USGS began its National High-Altitude Photography Program, 1:58,000-scale color-infrared photographs for the entire country became available; the National Wetlands Inventory uses these photographs extensively. In 1987, the USGS replaced the National High-Altitude Photography Program with the National Aerial Photography Program, which produces 1:40,000-scale color-infrared photographs; the National Wetlands Inventory uses these photographs as well. In some cases, the National Wetlands Inventory uses supplementary photography, such as some 1:60,000-scale color-infrared photographs of the prairie pothole region of the northern Great Plains, which were acquired from the National Aeronautics and Space Administration.

Stereoscopic color-infrared photographs are best for identifying and delineating wetlands. Color, texture, and pattern are important features of wetland vegetation and background soils. A combination of vegetation factors produce a specific response or signature on the photograph (Wilen and Pywell, 1992). These vegetation factors include leaf size, shape, structure, and arrangement; branching pattern; height; growth habit; and color. Determining the boundary of a wetland is the most difficult part of mapping. Normally, transitions are found at the boundary from upland vegetation to wetland vegetation, from nonhydric to hydric (wetland) soils, and from land that is not flooded to areas that are subject to flooding or saturation. On color-infrared photographs, water generally shows as a distinctive black and blue-black color because of its lack of reflectance. Wetlands that have canopy openings and contain standing water exhibit this signature along with assorted wetland-vegetation signatures. Saturated soils show darker tones because of the nonreflectance of the soil-water component. Even when wetland basins are dry, the silt, clay, and other fine-grained materials hold more water than the upland soils hold, which results in a distinctive dark color because of the lack of infrared reflectance.
The National Wetlands Inventory uses the best and most appropriate aerial photographs available for mapping wetlands.
Vegetation characteristics help to identify wetlands. Wetland vegetation generally is more dense, more crowded, and more concentrated than upland vegetation. Wetland vegetation normally exhibits a higher degree of lushness, vigor, and intensity than does upland vegetation. Even wheat grown in a dry wetland basin has a distinctive signature; it is more vigorous because of extra moisture in the basin. Dead and dying vegetation in flooded wetland basins also has distinctive signatures. When physiographic positions are associated with the vegetative characteristics described above, wetland locations become more obvious on an aerial photograph (fig. 47).

Figure 47

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Figure 47. Wetland features such as water, vegetation, and soil are identified on an aerial photograph by their signatures (left), and these signatures are used to product wetland maps (right). (Source: U.S. Geological Survey, 1995 (left); T.E. Dahl, U.S. Fish and Wildlife Service, unpub. data, 1992 (right).)

Patterns, or the repetition of the spatial arrangement, of vegetative types also provide important clues in the identification of wetlands. Basins that have a semipermanently flooded center may have a seasonally flooded band around the center and a temporarily flooded outer band. Patterns are not restricted to vegetation -- they can include drainage patterns and land-use patterns. Unplanted basins in farm fields might indicate wetlands; land-cover patterns such as ridges and swales help separate uplands and wetlands. When wetlands are being mapped, the photointerpreter closely checks areas indicated by swamp symbols as wetlands on USGS topographic maps and NRCS soil survey maps to ensure their possible inclusion as wetlands; such areas are considered wetlands unless strong evidence indicates otherwise.

A typical National Wetlands Inventory map consists of wetland boundaries added to a black-and-white version of a 1:24,000-scale USGS topographic base map. Wetlands are classified according to guidelines developed by Cowardin and others (1979). (See article "Wetland Definitions and Classifications in the United States" in this volume.) These wetland classifications are shown on the map as alpha-numeric codes that are identified in a map explanation at the bottom of the map. Many steps are involved in the production of a wetland map from selecting the sites for field verification to delineation, quality control, and production of the final map product (fig. 48). All National Wetlands Inventory photointerpreters are trained extensively in wetland identification, the FWS wetland classification system, and the field identification of wetland plants and soils in order to ensure the best quality, most accurate maps.

1. Determine project area.  
2. Obtain source materials.  
3. Prepare source materials. (photo A). fig48A
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4. Review photo interpretation and plan field trip. (photo B). fig48B
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5. Conduct a field reconnaissance of study area.  
6. Make photointerpretation .
(photo C)
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7. Check photointerpretation (quality control). (photo D) fig48D
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8. Transfer photointerpreted data to base map. (photo E) fig48E
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9. Check transferred information (quality control).  
10. Prepare copy of draft map for review.  
11. Conduct review of draft maps.  
12. Make changes to draft map manuscript. (photo F) fig48F
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13. Conduct final quality-control checks.  
14. Produce final map for distribution. (photo G) fig48G
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15. Digitize the final map. (photo H) fig48H
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Figure 48. The sequence of steps in producing National Wetlands Inventory maps. (Photographs A and E by Judy D. Fretwell, U.S. Geological Survey; all other photographs by Donald W. Woodard, U.S. Fish and Wildlife Service.)


Maps of the National Wetlands Inventory can be acquired from 33 State-run distribution centers, 6 USGS Earth Science Information Center regional offices, or by calling the USGS national toll-free number: 1-888-ASK-USGS. Maps can also be viewed at the Library of Congress and the Federal Depository Library System and downloaded cost-free through the National Wetlands Inventory Home Page on the Internet at The six regional USGS Earth Science Information Centers provide online computer links to the National Wetlands Inventory map data base, which contains current information about the availability and production history of National Wetlands Inventory maps and digital data. Digital data are available in Digital Line Graph 3 (DLG3) optional or Geographic Resources Analysis Support System (GRASS) formats; latitude and longitude, State Plane Coordinates, or Universal Transverse Mercator (UTM) coordinate systems; and 9-track, 8-mm, or 1/4-inch cassettes in UNIX-TAR or ASCII tape formats. Other products available at cost include acreage statistics by quadrangle, county, or study area and color-coded wetland maps.


References Cited

Cowardin, L.M., Carter, Virginia, Golet, F.C., and LaRoe, E.T., 1979,
Classification of wetlands and deepwater habitats of the United States: U.S. Fish and Wildlife Service, Biological Services Program Report FWS/OBS-79/31, 131 p.

Dahl, T.E., 1990,
Wetlands--Losses in the United States, 1780's to 1980's: Washington, D.C., U.S. Fish and Wildlife Service, Report to Congress, 21 p.

Wetland resources of the United States: U.S. Fish and Wildlife Service National Wetlands Inventory map, scale 1:3,168,000.

Frayer, W.E., Monahan, T.J., Bowden, D.C., and Graybill, F.A., 1983,
Status and trends of wetlands and deepwater habitats in the conterminous United States, 1950's to 1970's: Fort Collins, Colo., Colorado State University, 32 p.

Hall, J.V., 1991,
Wetland resources of Alaska: U.S. Fish and Wildlife Service National Wetlands Inventory map, scale 1:2,500,000.

Hall, J.V., Frayer, W.E., and Wilen, B.O., 1994,
Status of Alaska wetlands: Anchorage, Alaska, U.S. Fish and Wildlife Service, 33 p.

Metzler, K.J., and Tiner, R.W., 1992,
Wetlands of Connecticut: State Geological and Natural History Survey of Connecticut in cooperation with the U.S. Fish and Wildlife Service National Wetlands Inventory, Report of Investigations no. 13, 115 p.

Reed, P.B., Jr., 1988,
National list of plant species that occur in wetlands--1988 national summary: U.S. Fish and Wildlife Service Biological Report 88(24), 244 p.

Shaw, S.P., and Fredine, C.G., 1956,
Wetlands of the United States--Their extent and their value to waterfowl and other wildlife: U.S. Fish and Wildlife Service Circular 39, 67 p.
Teels, B.M., 1990,
Soil Conservation Service's wetland inventory, in Kiraly, S.J., Cross, F.A., and Buffington, J.D., eds., Federal coastal wetland mapping programs; a report by the National Ocean Pollution Policy Board: Washington, D.C., U.S. Fish and Wildlife Service Biological Report 90(18), p. 93-103.

Tiner, R.W., and Burke, D.G., 1995,
Wetlands of Maryland: Annapolis, Md., Maryland Department of Natural Resources, Water Resource Administration, in cooperation with U.S. Fish and Wildlife Service, National Wetlands Inventory, 193 p.

U.S. Fish and Wildlife Service, 1976,
Existing state and local wetland surveys (1965-1975), v. II, Narrative: Washington, D.C., U.S. Fish and Wildlife Service, Office of Biological Services Report, 453 p.

U.S. Geological Survey, 1995,
South Florida Satellite Image Map, 1993: Reston, Va., U.S. Geological Survey, 1 sheet, scale 1:500,000.

U.S. Soil Conservation Service, 1991,
Hydric soils of the United States: U.S. Soil Conservation Service in cooperation with the National Technical Committee for Hydric Soils, Miscellaneous Publication No. 1491, 3d ed., unnumbered pages.

Wilen, B.O., and Pywell, H.R., 1992,
Remote sensing of the Nation's wetlands, National Wetlands Inventory, in Proceedings: Forest Service Remote Sensing Applications Conference, 4th biennial, Orlando, Fla., unnumbered pages.

Wilen, B.O., and Tiner, R.W., 1993,
Wetlands of the United States, in Whignam, D.F., Dykyjova, Dagmar, and Hejny, Slavomil, eds., Wetlands of the world I--Inventory, ecology, and management: Dordrecht, the Netherlands, Kluwer Academic Publishers, p. 515-636.



Todd H. Votteler,
4312 Larchmont Avenue,
Dallas, TX 75205
Thomas A. Muir,
U.S. Geological Survey,
413 National Center,
Reston, VA 22092

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