NWI Program Overview

The National Wetlands Inventory (NWI) was established by the US Fish and Wildlife Service (FWS) to conduct a nationwide inventory of U.S. wetlands to provide biologists and others with information on the distribution and type of wetlands to aid in conservation efforts. To do this, the NWI developed a wetland classification system (Cowardin et al. 1979) that is now the official FWS wetland classification system and the Federal standard for wetland classification (adopted by the Federal Geographic Data Committee on July 29, 1996: 61 Federal Register 39465). The NWI also developed techniques for mapping and recording the inventory findings. The NWI relies on trained image analysts to identify and classify  wetlands and deepwater habitats from aerial imagery. NWI started mapping wetlands at a small scale (1:250,000 map which covers an area the size of 128-1:24,000 USGS topographic maps or approximately 7,400 square miles). Eventually, large-scale (1:24K scale) maps became the standard product delivered by NWI. As computerized mapping and geospatial technology evolved,  NWI discontinued production of paper maps in favor of distributing  data via on-line "mapping tools" where information can be viewed and downloaded. Today, FWS serves its data  via an on-line data discovery, the Wetlands Mapper. GIS users can access wetlands data through an on-line wetland mapping service or download data for various applications (maps, data analyses, and reports). The techniques used by  NWI have recently been adopted by the Federal Geographic Data Committee as the federal wetland mapping standard (FGDC Wetlands Subcommittee 2009). This standard applies to all federal grants involving wetland mapping to insure the data  can be added to the Wetlands Layer of the  National Spatial Data Infrastructure. NWI also produces national wetlands status and trends reports required by Congress.

Legislative Authorities and Administration Requirements

The Emergency Wetlands Resources Act of 1986 (P.L. 99-645, as amended) required the Secretary of the Interior, through the Director of the Fish and Wildlife Service, to map and digitize wetlands of the U.S. and to archive and distribute the data.  It further requires the Secretary to produce  national wetlands status and trends reports to Congress.  The Clean Water Act authorized for wetlands mapping to provide information to States to assist in the development and operation of programs under the Act.  The Fish and Wildlife Coordination Act authorized the Secretary to provide assistance to, and cooperate with Federal, State, and public or private agencies and organizations in the protection of all species of wildlife and their habitats and to make surveys of lands and waters of the U.S. and to accept funds for purposes covered in the Act.  The Office of Management and Budget Circular A-16 required the Department, through the FWS, to coordinate the Wetlands Layer of the National Spatial Data Infrastructure under the coordination of the Federal Geographic Data Committee (FGDC).  Other OMB Circulars also apply. The FWS is responsible for producing and maintaining the Wetlands Layer of the National Spatial Data Infrastructure (NSDI), which is a major component of the Department of Interior's geospatial line of business portfolio and E-government, and identified as a National Geospatial Data Asset (NGDA) by the FGDC on GeoPlatform.gov and Data.gov.

How Mapping Techniques Have Changed

The mapping techniques have also evolved over time. At the outset, NWI produced maps by first interpreting wetlands and deepwater habitats from high-altitude aerial photography (including 1:130,000, 1:80,000, and 1:62,500 scale photographs). Acetate overlays were attached to the aerial photos and the interpreter outlined and labeled wetlands and deepwater habitats with pen and ink. Data from the overlays were then transferred by cartographers to mylar overlays attached to a standard topographic map (e.g., 1:24,000 scale for lower 48 states and Hawaii, and 1:63,360 scale for Alaska). Small-scale maps were then made by cartographers through an engraving process. The final step would be digitizing data from large-scale NWI maps to create a geospatial database. As GIS and mapping technology advanced, the process of data collection and map production became an integrated operation (single step) done on-screen by the image analysts (Figure 1). These analysts delineated wetlands on-screen, and data were simultaneously entered into a digital data layer that could be used to generate maps at various scales using GIS technology. Today, all of the NWI data are created through this on-screen process. This technology also facilitated the use of other sources in the interpretation process as other digital datasets (e.g., USDA digital soil surveys and USGS digital topographic map information) could be viewed with the source imagery to identify wetlands.

How NWI Products Have Changed

Through most of NWI's history, large-scale wetland maps were the prime product (Figure 2). Today, the basic products are: 1) wetlands geospatial data that can be used to generate maps and information on the Nation's wetlands and 2) the national wetlands status and trends reports derived from data collected from a scientific monitoring study of wetland changes over time.

The NWI has also produced a variety of reports summarizing the results of its work for regional and local geographic areas. 

Digital wetlands data displayed on aerial imagery.
Figure 1. Digital wetlands data displayed on aerial imagery. Image interpretation using geospatial software has evolved from photo interpretation using stereoscope, pen and ink.
Example of 1:24K NWI map – Milton, DE quadrangle.
Figure 2. Example of 1:24K NWI map – Milton, DE quadrangle. (Note: Legend has been deleted for this figure)

Wetland Data Distribution

NWI geospatial data are available for decision makers for viewing or downloading via the Cloud (http://www.fws.gov/wetlands) and individuals can produce custom maps showing NWI data on planimetric maps (Figure 3). Data can be viewed on aerial imagery (Figure 4) or on topographic maps (Figure 5) through the National Map.

Wetlands data can also be downloaded or incorporated as a direct link by any organization through a web mapping service. Some applications incorporating NWI wetlands data include ESRI’s ArcGIS Resources Community; ArcGIS Online Resources; the FWS’ ECOS (Endangered Species planning); the US Army Corps of Engineers ORM2 (on-line wetland permitting system); and the Department of Housing and Urban Developments and over 60 social media portals.

The Federal Government is adopting a "Cloud-first" policy, in support of web-based computing as it looks to re-tool the way it buys information technology and OMB has required agencies to develop an analysis of how they could use cloud computing for all major technology projects. Working with FGDC’s Technology and Architecture Working Group, the NWI - Wetlands Mapper became the first geospatial service application to reach a Cloud computing platform from DOI. 

Example of custom NWI map made from the Wetlands Mapper.
Figure 3. Example of custom NWI map made from the "Wetlands Mapper." This map covers the area around Milton, DE - a portion of the area shown in Figure 2.
Google Earth screen capture.
Figure 4. NWI data displayed on imagery using GIS software for the Milton, DE area.
Topographic map for the Milton, DE area.
Figure 5. NWI data displayed on a USGS topographic map using the National Map Viewer for the Milton, DE area.

NWI Reports

The NWI has also prepared “special project reports” for work often funded by outside agencies including local and regional wetland trend reports and local wetland status reports (see section “Beyond Standard NWI Mapping – Special Projects”). Many of these reports are now on-line and can be searched on the NWI website using the “Documents Search Engine” (type in subject of interest); some may also be accessed on-line at the FWS' Conservation Library website. In addition to data summary reports the NWI has gone to great length to document technical procedures and data standards that are shared with partner organizations and the scientific community and available on the NWI website.

Why the National Wetland Status and Trend Study Was Started

Initially, expectations were that the inventory would be done in a few years given the inventory was a broad-brushed survey of wetlands (1:250,000 scale). However, it quickly became obvious that a national mapping program would be unable to sustain a complete, contemporary inventory of wetland resources. National wetland inventories are a powerful tool for conservation planners, however, these “wall to wall” type maps are a snap shot in time and are not easily updated, maintained or enhanced to meet the changing needs of resource managers. Yet, the continued pressures on the wetland resources required national policy and management questions about resource status rely on scientifically based processes to periodically measure wetland status and trends over time. Thus, wetland professionals rely on statistically based sampling strategies to capture current wetlands status and trends information. This sample based monitoring has been an effective means to gather information regarding wetland resources, A series of wetland status and trends reports describe the diversity of wetland types across the country and changes that have occurred over time., These reports educate policy-makers and the public on the status and threats to the Nation's wetlands and have influenced national and state policies and improved wetland conservation. National status and trend reports have been conducted every decade since as authorized by the Emergency Wetland Resources Act of 1986 (Dahl and Johnson 1991; Dahl 2000;Dahl 2006; Dahl 2011). The NWI receives cooperative funding from other agencies (Corps of Engineers, Environmental Protection Agency, Natural Resources Conservation Service, and National Oceanic and Atmospheric Administration-National Marine Fisheries Service) to conduct the assessments. The latest national trends study was completed in FY 2011. This report is the latest in a continuous series spanning 50 years of wetlands data. It represents the most comprehensive and contemporary effort to track wetlands resources at a national scale. The study found that, although much effort has been expended by the federal government and others to restore wetlands, the nation continues to lose large areas of natural wetlands. Please visit the Status and Trends web page for more information.   

Where We Are Today in Mapping the Nation's Wetlands

The maps below shows the status of the NWI across the country and the date of the imagery used to produce the data (Figures 6 and 7). Over the past 30 years, the NWI has produced wetland data for most of the county, with digital data available for about 81 percent of the country (continental United States = 100 percent).

As seen in Figure 7, the effective date of the NWI for most of the conterminous U.S. is 1980s (mostly derived from mid-1980s 1:58,000 color infrared photos). From 1991 to 1996, the NWI was producing data for 5% of the lower 48 states per year. Today, NWI data are being updated by the FWS at a rate of 2% per year with the help of outside partners. Since NWI is not producing wetlands data at the rate it was in its early years, a few states have taken the initiative to produce their own wetland inventory for priority areas. In July 2009, the Federal Geographic Data Committee established a federal wetland mapping standard that requires organizations receiving federal funds for such efforts to follow; this will ensure that new wetland data produced with federal dollars will be collected in a format compatible with the national wetlands geospatial database (Federal Geographic Data Committee 2009). If maps are produced solely using State funds, these standards do not apply, so quality control and conversion of non-standard data may be required.

Status map
Figure 6. Status of NWI data for the country as of May 2014.
Map of era of imagery used for NWI.
Figure 7.  Era of imagery used for mapping NWI wetlands and deepwater habitats as of May 2014.

Who Has Contributed to the NWI? 

Over the past 37 years, the NWI has had many cooperators in conducting the inventory, a national asset worth $220M. An acknowledgment of mapping cooperators appears on our Data Contributors website. Some partners have provided funding to perform the surveys, some conduct the surveys, some participate by reviewing draft NWI data, while others contribute data to the NWI. Cooperators include states, other federal agencies, tribal governments, regional and local governments, and nonprofit organizations. We have also received funding from other FWS programs to map wetlands including the Prairie Pothole Joint Venture, Partners for Fish and Wildlife, and the National Wildlife Refuge Program. Funding partners in 2012 included New Mexico, Connecticut, the FWS’ National Wildlife Refuge System, the Prairie Pothole Region Landscape Conservation Cooperative, and the US Geological Survey. Reimbursable funding for direct map production from outside sources has averaged $0.6M over the prior three years, while other FWS programs have contributed an average of $0.4M over the same period. Outside funding has remained low as the amount of contributed data has increased. Partner organizations have historically been willing to step-up and either provide in-kind services for data reviews knowing that FWS as data stewards would provide for national consistency, quality control and/or data conversions to ensure data integrity as part of a National data repository. There is strong sentiment among partner organizations that this is inherently a Federal role in the formulation of a nationally recognized, authoritative data set such as the Wetlands Data Layer. Many partner organizations struggle with funding issues but are willing to contribute some services or data.   

How the NWI Budget Has Changed

The NWI budget has risen and fallen since 1975 and has flattened at around $5M (Figure 8). In 1986, NWI received roughly a $1M boost, while in 1992, a $3M increase occurred largely as OMB provided additional funding to support the wetlands status and trends study. In 1996, the NWI mapping budget was reduced by about 50% which severely hampered the NWI’s ability to keep maps up-to-date and to produce original mapping for unmapped areas. For FY 2013, the NWI budget is $4.2 million. Nearly $3.2M is required to pay salary, space, equipment, and administrative costs to keep the NWI running, leaving $1M for projects before possible budget sequestration. The NWI has identified over $100 million in projects for updating areas where current data are needed to meet FWS priorities.

Graph showing appropriated funding for NWI.
Figure 8. Appropriated funding for NWI since 1975.
Graph of funding for NWI including reimbursable funding.
Figure 9. Funding for the NWI including reimbursable funding from other sources since 1975. Dollar amounts are in thousands of dollars (e.g., $8,000 = $8,000,000). This graph does not include funds secured by individual regions for NWI mapping and special projects where contracts were processed through the regions. (Note: These figures do not include reimbursable funding received by the FWS from other federal agencies for two national wetland status and trend studies ($1.8M in 2004-5 and $800K in 2008-9).

How NWI Data Are Used Across the Country

Habitat degradation and destruction continue combined with emerging conservation issues related to flooding and coastal resilience, global climate change (including sea-level rise, flooding, and drought), and domestic energy development have heightened the need for updated wetlands data. Applications of NWI data include use in: 1) predicting the impacts of sea-level rise, 2) wetland restoration planning, 3) planning for energy independence (primarily in the West and Alaska), 4) analyzing carbon sequestration in wetlands, 5) planning, management, and performance reporting for National Wildlife Refuges (including targeting areas for acquisition) and other federal lands, 6) planning, modeling, research, and monitoring for Strategic Habitat Conservation and surrogate species efforts by the Service and Landscape Conservation Cooperatives, 7) recovery planning for endangered species, fish, migratory birds, marine mammals, and other imperiled species, and 9) invasive species control. General uses of NWI data are summarized below.

Wetland Protection and Management

NWI data integrated into Corps' permit tracking system map.
Figure 10. NWI data are integrated into the Corps’ wetland permit tracking system. Green areas = NWI wetlands, blue areas = NWI deepwater habitats, and pink and white areas = Corps field-verified wetlands in a project area. The latter wetlands were mostly below the target mapping unit; by comparison, the dark green wetland below I-94 is 3.3 acres in size.

The general public consults NWI data on a daily basis via the NWI website (Wetlands Mapper) when considering land purchases and development. Landowners, developers, real estate agents, and environmental consultants review NWI data as a first step in assessing the potential restrictions of land for residential, commercial, and industrial development. The U.S. Army Corps of Engineers (Corps) uses NWI data during its permit review process (e.g., cumulative effects determinations and potential sites for mitigation banking) and includes a link to NWI data on its on-line permit tracking system (Figure 10). Pesticide applicators use NWI data to reduce the effects of their products on wetland habitats.

Climate Change Impact Analysis

An important use of NWI data has arisen from concern about the impacts of climate change. The Sea-Level Affecting Marshes Model (SLAMM) predicts the possible effects of sea-level rise on coastal wetlands and adjacent lowlands. This model uses NWI data plus information on local topography, accretion and erosion rates, dikes, and development in making these predictions. SLAMM has been run for coastal South Carolina and Georgia, a few large estuaries including Puget Sound, Chesapeake Bay, and Delaware Bay and for numerous National Wildlife Refuges along the Atlantic, Gulf, and Pacific Coasts (Figure 11). The FWS plans to apply SLAMM to all coastal refuges to aid in planning acquisition and management and added 66 additional refuge simulations to SLAMM-View in FY 2012 [hosted on-line by the FWS’ Chesapeake Bay Field Office (http://www.fws.gov/slamm/)]. Since a rising sea level threatens the integrity of dikes forming coastal waterfowl impoundments, refuge managers are using SLAMM and other data to formulate a strategy for determining when to continue maintaining the dikes and when to abandon such structures.

Climate change will also pose significant consequences for inland wetlands with predicted shifts in precipitation patterns. NWI data are being used to quantify loss of wetlands in the prairie pothole region attributed to predicted drought in the upper Midwest. These declines will affect populations of waterfowl, shorebirds, and endangered species, and also could have a major impact on farmers, food security, and biofuels development. Since wetlands store carbon, wetland restoration can be important in reducing carbon dioxide concentrations in the atmosphere. NWI data have been used to help quantify carbon stored on wetlands. Restored farmed wetlands may form the basis for carbon-credit trading to mitigate greenhouse emissions from other sources that affect global climates. Existing NWI data serve as a starting point to identify potential restoration sites (farmed wetlands and former wetlands) as well as to monitor carbon sinks for purposes of meeting future carbon sequestration needs.

SLAMM use of NWI data example.
Figure 11. SLAMM uses NWI data to identify wetlands and marsh elevations for predicting and displaying the effects of sea-level rise in coastal regions: Year 2000 conditions (left) & Year 2100 simulation (right); note the inundation of former coastal marshes in the year 2100 simulation.

Emergency Planning and Recovery

Mapper developed in response to the Deepwater Horizon oil-spill.
Figure 12. Mapper developed in response to the Deepwater Horizon oil-spill showing the location of fish and wildlife resource areas in the Gulf of Mexico along with oil plume and shoreline oil locations.

The Federal Emergency Management Agency (FEMA) and other federal and state agencies use NWI data to assist in planning, recovery, remediation, and mitigation for hurricane and other flooding disasters. For example, after hurricane Katrina, the NWI partnered with the U.S. Geological Survey’s National Wetlands Research Center to convert existing NWI maps to digital data and update the information for Louisiana for use by federal, state, and local agencies in recovery and planning efforts. This information would also be used for a myriad of other purposes to help protect wetlands and wetland-dependent or associated wildlife, fish, and plant species. NWI data are expected to be used for federal inter-agency planning for coastal resiliency and natural flood adaptation or abatement under the Presidential Policy Directive #8, National Preparedness, for the mitigation and recovery frameworks, assisting FEMA under the Mitigation Framework Leadership Group (MitFLG) and the Technical Mapping Advisory Council, and providing information for coastal communities for preparedness planning; and to support the efforts of the Federal Inter-agency Floodplain Management Task Force to implement Executive Order 11988 on Floodplain Management to reduce the effects of flooding and protect the natural benefits of floodplains.

NWI data have been used in other emergency situations. NWI supported FWS and other Federal agencies efforts in the response to the Deepwater Horizon oil-spill disaster, developing a fish and wildlife resource areas mapper within days of the disaster that included wetlands among the data layers for oil-spill response (Figure 12). The FWS responded to a request from the Center for Disease Control and the U.S. Department of Agriculture’s Centers for Epidemiology and Animal Health (APHIS) related to avian influenza (bird flu). Because of the human health threats posed by a potential pandemic, infectious disease specialists wanted to develop a model to assess the threat level based on transmission of avian influenza virus spread through wild populations of migratory waterfowl and other water birds. To conduct these analyses, disease specialists needed to know where and how many wetlands and surface water bodies might serve as staging areas for migrating birds.

The FWS provided its entire wetlands digital dataset for the conterminous United States (40+ gigabytes) for the avian influenza study. APHIS used this information to develop models of areas susceptible to avian influenza outbreaks and possible consequences for the deployment of emergency response medical teams and supplies. NWI data were also used by NASA for shuttle debris recovery operations following the Columbia disaster.

Strategic Habitat Conservation

Since an estimated 46% of endangered or threatened species are associated with wetlands, NWI data (including riparian habitat data for western states) are being used to help determine occurrence of species and design plans for species recovery. Recently, the FWS has received a petition to list another 404 aquatic and aquatic dependent species in the Southeast U.S.  Refined wetlands data can support listing, planning or recovery. NWI wetland classification types are used to identify potential habitat for some species, or could be used in combination with other data to locate such habitat. The FWS will need wetlands data for its Strategic Habitat Conservation adaptive management and its surrogate species performance reporting approach for planning and monitoring fully-functioning landscapes together with needed upland mapping efforts. FWS supported Landscape Conservation Cooperatives will use these data for planning and modeling for wildlife habitats.

Strategic Habitat for Mussels and related aquatic species in Alabama map.
Figure 13. Strategic Habitat for Mussels and related aquatic species in Alabama.

Supporting Strategic Habitat Conservation – Alabama’s Aquatic Resource Initiative

The loss of aquatic biodiversity and habitat is now considered a major form of global change. Environmental degradation and the loss of biodiversity have led to new approaches in conservation, management and restoration. The FWS (AL Ecological Services Field Office and NWI), in cooperation with the Alabama Department of Conservation and Natural Resources - Aquatic Biodiversity Center, the Geological Survey of Alabama, and the Alabama Clean Water Partnership have initiated a project that will provide geospatial context for biological planning and conservation. Initial actions include the development of a public facing geospatial database outlining Strategic Habitat Units (SHUs) in Alabama and describing areas essential to the survival of more than 200 imperiled aquatic species (mussels, snails, fish, crayfish) (Figure 13); enhance aquatic habitat and riparian mapping in key SHUs; and deploy a secure interactive data management viewer for partners to report field activities and prioritize management actions. The goal is to enhance species recovery opportunities at the landscape scale across the state of Alabama. This can then be applied to guide conservation actions for aquatic species at risk region or nation-wide.

Waterfowl Management

Waterfowl management is a major focus of the FWS and the Prairie Pothole Region (PPR) is the Nation’s premier waterfowl production area. NWI data have been used to produce breeding pair accessibility maps for the PPR. These maps display predictions of the number of upland nesting duck pairs that could potentially nest in the upland portion of every 40-acre block of the PPR of Minnesota and Iowa. These predictions are based on the known maximum travel distances of hens from wetlands to their nest sites and regressions (statistical models) created from four-square mile survey data predicting the number of duck pairs that utilize every individual wetland in the PPR during a “typical” breeding season (Figure 14). The maps are used to help identify priority sites for the protection or restoration of grassland habitats for breeding waterfowl, but are also useful in identifying priority wetland complexes to be protected through acquisition and easements, or to be enhanced by private lands wetland restorations. The U.S. Department of Agriculture’s Natural Resources Conservation Service uses NWI data to respond quickly and accurately to farmers’ requests for wetlands determinations in this vital waterfowl area. With almost 50 percent of North American migratory birds associated with or dependent on wetlands, sixteen states, Guam, and the Virgin Islands have identified NWI data as needed information for wildlife conservation planning (GA, IL, KS, MA, MI, MN, MT, NV, NH, NM, NY, OH, PA, TX, VA, and WV). In addition, officials under the North America Waterfowl Management Plan, Joint Ventures, and Ducks Unlimited have stated their need for modernized wetlands data and support for the NWI.

 NWI data used to conduct migratory bird surveys maps.
Figure 14. NWI data are used in conjunction with migratory bird surveys to identify bird density in various habitats: breeding pairs in the Prairie Potholes Region (left) and marsh bird habitat preferences as part of a nationwide survey (right).

Determining Wetland Density at the Landscape Level for the Nation

Strategic Habitat Conservation practices hinge on integrating biological and geospatial information at the landscape level to achieve conservation objectives.  Technological advances in geospatial data capture and management continue to change and improve the way biological planning, inventory and assessments are conducted. Geospatial wetland map data combined with other biological information are important decision support tools as part of the Strategic Habitat Conservation approach. The FWS’ habitat conservation actions will increasing rely on geospatial  habitat and trend information to help guide, prioritize, and assess species recovery, wildlife resource management, wetland threats and habitat restoration project actions.    A map of the current distribution and relative density of wetlands in the conterminous United States has been compiled by NWI to assist managers and resource specialists in understanding regional patterns. The information has been interpolated from recent data from the NWI's wetlands status and trends study and reflect current density of wetland resources at a landscape-level. This representation at 1:5,000,000 scale is useful to further guide surface water and wetlands research, management and policy. 

Wetland density of the conterminous United States map.
Figure 15. Wetland density of the conterminous United States.

Aid to Other Mapping Efforts

The NWI data have been used by numerous agencies and academic institutions as base data for identifying wetlands for a host of remote sensing applications. Data have been used to assist investigators in performing supervised and unsupervised classification of wetlands as part of regional or national inventories of land use and land cover. For example, NOAA’s C-CAP Program produces data and maps using NWI data to aid in detecting wetlands on satellite imagery. Similarly, university researchers have used NWI data in testing local or site-specific applications of remote sensing technologies. NWI data has provided a foundation for more detailed wetland mapping in some states and local areas. For example, the State of Delaware relied heavily on NWI data when updating wetland data for a statewide wetland database.    

GeoMine Project

The Office of Surface Mining Reclamation and Enforcement (OSMRE) is engaged in a project to build and modernize data, databases and data delivery tools to address coal mining in the Appalachian region of the United States. The USFWS National Wetlands Inventory (NWI) was asked to participate in the early stages of development of this project based on their expertise in large databases, distributed data contributors and on-line mapping technologies. The NWI provided data design and collection recommendations, a copy of a Web-based Mapper and Web Mapping Services for the NWI Wetlands data for incorporation into their GeoMine viewer. Wetland data analysis was also conducted to provide OSMRE with the quantity and acreage of wetlands and deepwater habitats within the GeoMine project areas.

Map showing wetlands and deepwater habitats.
Figure 15. Wetland density of the conterminous United States.

Beyond Standard NWI Mapping - Special Products

The NWI through existing staff have the skills and technology to do more than produce standard NWI data and wetland status and trends analyses. To meet the needs of our cooperators for additional wetland and related habitat information, we have developed a few other products. These products include riparian habitat classification and mapping, surface waters and wetlands mapping, and specialty mappers for the Coastal Barrier Resources System.

Riparian Habitat Classification and Mapping

In the regions where evaporation exceeds precipitation, riparian habitats are as critical for wildlife as wetlands are in the more humid regions. As much as 80 percent of wildlife species in these regions depend on riparian habitats for breeding, foraging . Such habitats are important migration corridors. The condition of riparian habitats is also important for maintaining healthy aquatic systems. Given these well-recognized values, the NWI felt it would be beneficial to include these habitats in its inventory in areas west of the Mississippi River. To standardize this mapping, the NWI developed a riparian classification system and mapping conventions (U.S. Fish and Wildlife Service 1997; 2009). This classification has been used in combination with the FWS’s wetland classification system to produce NWI maps showing both riparian areas and wetlands in the arid regions of the country (Figure 17).

Portion of an NWI map showing wetlands and deepwater habitats.
Figure 17. Portion of an NWI map showing wetlands in green, deepwater habitats in blue and riparian habitats in tan.

Mapping Historic Wetland

Historic wetlands and current wetlands for an area near De Smet, SD.
Figure 18. Historic wetlands (blue) and current wetlands (green) for an area near De Smet, SD

The FWS’ habitat conservation actions increasingly rely on geospatial habitat and trend information to help guide, prioritize, and assess species recovery, wildlife resource management, wetland threats and habitat restoration project actions.  Therefore the development of historic wetland data is useful for a variety of reasons including landscape level planning and modeling, determining possible wetland reestablishment opportunities and tracking changes in land use.

The NWI has formulated a historic wetlands data layer defined as areas where there is evidence that a wetland once existed.  This evidence can be from historical map information, inventories of past wetland extent or other information collected that relate directly to data on wetland filling, drainage or other modifications.  Areas not included as historic wetland include topographic depressions where there is no evidence that hydrology created wetland conditions; modeling of potential past wetland extent based on empirical datasets; or non-geospatial data.  The term “historic wetland” is not synonymous with “restorable wetland” as used here.  Wetland restoration or reestablishment is dependent on a number of factors including past land treatments (filling, flooding or land leveling), current land use and changes in hydrology. Although the reestablishment of some historic wetlands may be improbable, their tracking can be valuable in assessing regional changes in hydrology and other ecological trends.  

National Wetlands Inventory – Version 2, Surface Waters and Wetlands Dataset

Wetlands, an essential component of the nation’s surface water network, are closely linked with hydrology either through exchange of water, nutrient cycling, or other ecological processes. Hydrological data models have been created to represent water flow for geospatial use, though models without all surface water features are incomplete Hydrologic models used for ecological or landscape level applications should include wetland geospatial data, as it is essential to understand the interactions between surface waters and wetlands. To this end, NWI has developed a more comprehensive dataset (NWI Version 2) that is inclusive of all wetlands and surface water features. This dataset will help focus efforts on providing the base data for geospatial models designed to examine linkages between surface waters and wetlands.

Understanding the biodiversity values associated with different surface water features is an important factor in achieving many strategic conservation goals. The synthesis of this water resource information is crucial to an array of users and aids efforts to produce national thematic map-based products that may assist in predictive, multi-scale, and system-focused actions for resource assessment.

Since the Supreme Court decision of 2001 (SWANCC) established a distinction between isolated wetlands and wetlands with connections to waterways for federal regulatory purposes, determining landscape hydrologic connectivity has taken on added significance and regulatory ramifications. Although regulatory jurisdiction is determined on a case by case basis, the NWI Version 2 dataset provides additional information to help identify and quantify isolated wetlands.

A comparison of mapped water bodies around Lake Henry, Kingsbury County, SD.
Figure 19. A comparison of mapped water bodies around Lake Henry, Kingsbury County, SD. This graphic shows the open water bodies (black) and associated wetlands (shades of gray) between the NWI Version 2 data (left image) and the National Hydrography Dataset (right image). Legacy NWI data were incorporated into the NWI Version 2 dataset because it provided greater accuracy of littoral/limnetic boundaries; a consistent, standardized classification (classification attributes not shown in this example) and greater resolution of surface water boundaries.
- Last updated: May 4, 2020 -