This EnviroAtlas dataset is a digital hydrologic unit boundary layer to the Subwatershed (12-digit) 6th level for the conterminous United States, based on the January 6, 2015 NHDPlus V2 WBD (Watershed Boundary Dataset) Snapshot (NHDPlusV21_NationalData_WBDSnapshot_FileGDB_05). The feature class has been edited for use in for EPA ORD's EnviroAtlas. Features in Canada and Mexico have been removed, the boundaries of three 12-digit HUCs have been edited to eliminate gaps and overlaps, the dataset has been dissolved on HUC_12 to create multipart polygons, and information on the percent land area has been added. Hawaii, Puerto Rico, and the U.S. Virgin Islands have been removed, and can be downloaded separately. Other than these modifications, the dataset is the same as the WBD Snapshot included in NHDPlus V2. This dataset was produced by the US EPA to support research and online mapping activities related to EnviroAtlas. EnviroAtlas (https://www.epa.gov/enviroatlas) allows the user to interact with a web-based, easy-to-use, mapping application to view and analyze multiple ecosystem services for the contiguous United States. The dataset is available as downloadable data (https://edg.epa.gov/data/Public/ORD/EnviroAtlas) or as an EnviroAtlas map service. Additional descriptive information about each attribute in this dataset can be found in its associated EnviroAtlas Fact Sheet (https://www.epa.gov/enviroatlas/enviroatlas-fact-sheets).