This data layer represents all surface water features in the United States. This grid was created by combining water features identified in two sources, the 2019 National Land Cover Database (NLCD), and the National Hydrography Dataset (NHD Plus version 2.1). First, the surface water features were extracted from the 2019 NLCD produced by the Multi-Resolution Land Characteristics Consortium. Features included are 'Open Water' (code 11), 'Woody Wetlands' (code 90) and 'Emergent Herbaceous Wetlands' (code 95).The second source used was the flowline and waterbody features as represented in the catseed grid from the NHD Plus version 2.1. The combination of these two datasets represents surface water and is referred to as the 'Water Mask.' This dataset was produced by the US EPA to support research and online mapping activities related to the Watershed Index Online (WSIO) and 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).