The Hydrologically Connected Zone (HCZ) Mask was determined using grid analysis to combine surface water features in the United States from three datasets: First, the surface water features from the 2011 CDL-NLCD. The CDL-NLCD, which was developed by the EnviroAtlas team, is a hybrid of the 2011 CDL produced by the National Agricultural Statistics Service (NASS) and the 2011 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). Second, the flowline and waterbody features as represented in the catseed grid from the National Hydrography Dataset (NHD) Plus version 2.1, downloaded June 18, 2013. Third, all areas contiguous to surface water that also have a wetness index value of 550 or greater. The wetness index, also known as the compound topographic index (CTI), is a steady state wetness index. It is commonly used to quantify topographic control on hydrological processes. The combination of these three datasets represents the Hydrologically Connected Zone. This dataset was produced by the US EPA to support research and online mapping activities related to EnviroAtlas and the Watershed Index Online (WSIO). 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).