Keywords: Water Resource Regions of the United States of America.jpg en Map of the 21 water resource regions with unique 2-digits hydrologic unit codes HUCs of the United States of America per the USGS hydrologic unit classifications of region as the first level of classification 2009-11-09 06 28 43 http //water usgs gov/wsc/imageregions jpg USGS other versions PD-USGov Uploaded with UploadWizard Geography of the United States Hydrology in the United States United States Geological Survey maps Basins of the United States Maps of rivers of the United States |