The Hydrologic Unit Code (HUC) is part of a hierarchical classification system for surface water drainage in the US (Seaber et al. 1987). The numerical code represents a cataloging unit of delineation. Vector format (i.e. polygon) data were compiled from United States Geological Survey (USGS) source - the Watershed Boundary Dataset (WBD). In order to include areas of large water bodies and ocean shorelines, an iterative process was used to expand polygons boundaries 5 kilometers extending from landward edges. This altered 12-digit code HUC dataset was used to delineate species ranges as part of the USGS Gap Analysis Project's species range and distribution modeling effort.