National Wetlands Inventory
Ecological Services

Product Summary - Historic Wetlands Data Layer

The U.S. Fish and Wildlife Service (Service) is the principal Federal agency that provides information to the public on the extent and status of the Nation's wetlands and provides stewardship for the wetlands data that comprise the Wetlands Layer of the National Spatial Data Infrastructure.  

In 2006, the Service released a Strategic Habitat Conservation plan to guide agency resource management decisions. 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 ecological information are important decision support tools as part of the Strategic Habitat Conservation approach. The Service’s habitat conservation actions will 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.  Historical wetlands are areas where there is evidence a wetland once existed.  This information is useful for a variety of reasons including landscape level planning and modeling, determining possible wetland reestablishment opportunities and tracking changes in land use or ecological trends.

Historic Wetlands Data  

Historic wetlands have been identified using several different techniques depending on the availability and type of information used to locate these areas and user needs.  For example, historical maps often provide information about past wetland extent or location and can be useful tools to identify historic wetlands (Figure 1).  Similarly, aerial photographs can provide a historical record of wetland extent.  More recently, the use of some databases (i.e., areas mapped as having wetland soil types in the soil survey) have been used in attempts to identify historic wetland.  All of these methods have limitations in their ability to locate, identify and quantify historic wetland areas.   In the formulation of this data layer, historic wetlands are 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.  

Historic topographic map example.
Figure 1.  Historic topographic map information indicates wetland extent (swamp symbology) along the shoreline of Green Bay, WI in 1954 (Source: USGS, Topographic 7.5 minute map series, Green Bay West Quadrangle, 1954).

Identification and Attribution of Historic Wetlands  

Historic wetlands have been identified as polygonal data.  No linear features (lines) have been included.  Historic wetland polygons are not classified or attributed with wetland labels (Figure 2).  Since these features no longer exist, boundary delineations are considered approximations based on topography, previously mapped information or indications of historic water levels.

Historic wetlands polygins example.
Figure 2.  Historic wetland polygons (shown in blue) are not classified or labeled since they are historic features.  They are displayed here on the Wetlands Mapper in combination with existing mapped wetlands (shown in green).

Historic Wetland versus Restorable Wetland  

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.  Some recent studies have demonstrated that some wetlands may not be restorable because of landscape-level changes to hydrology.   Efforts to reestablish wetlands have been focused on less intensively developed land (i.e. agricultural lands) or on undeveloped land.  Only rarely have wetlands been reestablished in intensively developed areas such as shown in Figure 3.

Evidence of lost wetlands photo.
Figure 3.  Evidence of lost wetlands (outlined in red) that have been developed as upland.  The opportunity to reestablish wetland in such areas is improbable, but tracking these land use changes can be useful for assessing regional changes in hydrology, and other ecological trends.

Please read the DisclaimerData Limitations, Exclusions and Precautions, and the Wetlands Geodatabase User Caution.

Last updated: May 10, 2017