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This data set represents the extent and approximate location of historic wetland habitats in certain areas of the conterminous United States. The identification of these historic wetlands is limited by the methodology outlined in the associated Historic Wetlands Project Metadata and is not intended to be a comprehensive inventory of all historic wetlands. 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 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. 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. Historic wetlands have been identified as polygonal data. No linear features have been included. Historic wetland polygons are not classified as wetlands and have no wetland labels or attribution. Since these features no longer exist, boundary delineations are considered approximations based on topography, previously mapped information or indications of historic water levels.
The Watershed Boundary Dataset (WBD) is a comprehensive aggregated collection of hydrologic unit data consistent with the national criteria for delineation and resolution. It defines the areal extent of surface water drainage to a point except in coastal or lake front areas where there could be multiple outlets as stated by the "Federal Standards and Procedures for the National Watershed Boundary Dataset (WBD)" “Standard” (http://pubs.usgs.gov/tm/11/a3/). Watershed boundaries are determined solely upon science-based hydrologic principles, not favoring any administrative boundaries or special projects, nor particular program or agency. This dataset represents the hydrologic unit boundaries to the 12-digit (6th level) for the entire United States. Some areas may also include additional subdivisions representing the 14- and 16-digit hydrologic unit (HU). At a minimum, the HUs are delineated at 1:24,000-scale in the conterminous United States, 1:25,000-scale in Hawaii, Pacific basin and the Caribbean, and 1:63,360-scale in Alaska, meeting the National Map Accuracy Standards (NMAS). Higher resolution boundaries are being developed where partners and data exist and will be incorporated back into the WBD. WBD data are delivered as a dataset of polygons and corresponding lines that define the boundary of the polygon. WBD polygon attributes include hydrologic unit codes (HUC), size (in the form of acres and square kilometers), name, downstream hydrologic unit code, type of watershed, non-contributing areas, and flow modifications. The HUC describes where the unit is in the country and the level of the unit. WBD line attributes contain the highest level of hydrologic unit for each boundary, line source information and flow modifications.