This data set is a complete digital hydrologic unit boundary layer of the Subbasin (8-digit) 4th level for the entire United States, clipped to the Northeast.

A hydrologic unit (HU) is a drainage area delineated to nest a multi-level, hierarchical drainage system. Its boundaries are defined by a hydrographic and topographic criteria that delineate an area of land upstream from a specific point on a river, stream, or similar surfaces waters. An HU can accept surface water directly from upstream drainage areas, and indirectly from associated surface areas such as remnant, non-contributing, and diversions to form a drainage area with single or multiple outlet points. HUs are only synonymous with classic watersheds when their boundaries include all the source area contributing surface water to a single defined point.

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 Dataset; 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.nrcs.usda.gov/wps/portal/nrcs/detail/national/water/watersheds/?cid=nrcs143_021630 . Polygons are attributed with hydrologic unit codes, name, and size. The data is currently updated through the USGS National Hydrography Dataset (NHD) Program and replicated to NRCS twice per year.