USGS Hydrologic Unit Codes
- Newest List of Hydrologic Units: Watershed Boundary Dataset (WBD) [643 kb, pdf]
- Boundary Descriptions and Names of Regions, Subregions, Accounting Units and Cataloging Units (outdated) [803 kb, pdf]
- Hydrologic Unit Maps, USGS Water-Supply Paper 2294 (outdated) [12 MB, pdf]
- USGS Watershed Finder (Note: Mid-Atlantic  and Great Lakes  boundaries are outdated)
HUC Data Files
- Gridded WBD Hydrologic Codes netCDF [37 kb, nc]
- Gridded WBD Hydrologic Codes Tabular [107 kb, txt]
- Gridded WBD Hydrologic Codes Maps [648 kb, pdf]
Drainage Basin Lookup Tables
HUC 2 is regional, HUC 4 is sub region, and HUC 6 is basin. These have headings of an ID number, lon and lat point, the HUC region number it belongs to (in either 2, 4, or 6 digits), the name of that region, the states which fall into that region, the shape length and area, and some more ID information. The lat/lon points outside the US are still included but do not have any other attributes associated with them.
The next 3 files include the lat/lon points that are joined to both the USGS divisions and the Canadian divisions in one large table. This data does have a different resolution than the USGS data, and was gathered from Natural Resources Canada. The column headings here are a little more complex so there is a "variable name" sheet within each of these files to keep track of them.