The following is extracted from USDA, NRCS metadata -- "This data set, part of the Watershed Boundary data set (WBD), is a complete digital hydrologic unit boundary layer to the Subbasin (8-digit) 4th level for the entire United States. This data set consists of geo-referenced digital data and associated Attributes created in accordance with the "Federal Guidelines, Requirements, and Procedures for the National Watershed Boundary data set; Chapter 3 of Section A, Federal Standards, Book 11, Collection and Delineation of Spatial Data; Techniques and Methods 11-A3" (04/01/2009). http://www.ncgc.nrcs.usda.gov/products/data sets/watershed/index.html. Polygons are Attributed with hydrologic unit codes for 1st (Region), 2nd (Sub-Region), 3rd (Basin), and 4th (Sub-Basin) Hydrologic Unit Level codes, names, Sub-Basin acres and square miles."