Historical wetland data can be very useful for a variety of reasons including landscape level planning and modeling, accessing wetland reestablishment opportunities and tracking changes in land use. Historic wetlands have been identified by the Service using several different techniques depending on the availability of information within a given area. 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 these wetlands. 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 wetlands displayed on the Wetlands Mapper represent 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. Please note that currently historic wetland data are only available in certain areas of the U.S. Where available, these data can be download by state or watershed.
The term “historic wetland” is not synonymous with “restorable wetland." 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 due to 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. It is extremely rare for wetlands to be reestablished in intensively developed areas such as shown in the image below (Figure 3).