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 Subwatershed (12-digit) 6th 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 4th level subbasins, 5th level watersheds, 6th level subwatersheds, name, size, downstream hydrologic unit, type of watershed, noncontributing areas and flow modification."