What are Hydrologic Unit Codes (HUCs)?
The Watershed Boundary Dataset (WBD) contains Hydrologic Units for the United States. The largest unit of measurement is a Region. Regions have a two-digit hydrologic unit code (HUC). For example, the Upper Colorado Region has a two-digit HUC of 14. Region 14 contains several smaller Hydrologic Units. These are known as Subregions and have a four-digit HUC. Subregion 1401 for instance, is the Colorado Headwaters Subregion and is contained within region 14. Subbasins are the third level of measurement and have an eight-digit HUC. Hydrologic Units are broken down to the smallest unit of measurement in the WBD known as a Subwatershed. Subwatersheds have a twelve-digit HUC. When you open the NHD Viewer you will see that each Hydrologic Unit is labeled with its HUC.