Hydrologic Unit Codes (HUCs) Explained
Watersheds are delineated by USGS using a nationwide system based on surface hydrologic features. This system divides the country into 22 regions (2-digit), 245 subregions (4-digit), 405 basins (6-digit), ~2,400 subbasins (8-digit), ~19,000 watersheds (10-digit), and ~105,000 subwatersheds (12-digit). A hierarchical hydrologic unit code (HUC) consisting of 2 additional digits for each level in the hydrologic unit system is used to identify any hydrologic area (see Federal Standards and Procedures for the National Watershed Boundary Dataset). A complete list of Hydrologic Unit codes, descriptions, names, and drainage areas can be found in the United States Geological Survey Water-Supply Paper 2294, entitled "Hydrologic Unit Maps".
What do the numbers represent?
Each hydrologic unit is assigned a 2-digit to 12-digit number that uniquely identifies each of the six levels of classification within six two-digit fields. An example is shown below using hydrologic unit code 180902030303:
More sites with information about HUCs:
How's My Waterway? v1.0
Map a watershed
USGS Water Resources Division - Hydrologic Unit maps and spatial data
USGS Watershed Boundary Dataset
USGS National Hydrography Dataset
USGS GIS Data for Water Resources
Water Topics - EPA Office of Water
USGS Geographic Names Information System (GNIS) - here you can find the location of a lake, have it mapped, and through a link to the EPA Surf your watershed site, find out what HUC it is in. A great tool; we use it all the time.
Hydrological code - Wikipedia