Source: US EPA, NRCS, USGS
Date: Last Updated 5/08
NRCS and USGS are developing a nationwide digital watershed boundary dataset. The dataset is comprised of nested regions, called hydrologic units (HUs), which delineate progressively smaller watersheds. Each hydrologic unit has a code assigned to it, called a hydrologic unit code (HUC). A HUC is a series of two-digit groupings of numbers that describe a hydrologic unit’s scale, plus where it fits in the larger hydrologic unit framework.
Hydrologic units range in size from regions, which can cover several states, to subwatersheds, which generally cover areas of 25 to 50 square miles. HUCs start with a two-digit code for the region level, and then additional two-digit codes are appended as one moves in to smaller watersheds. The subwatershed level contains a 12-digit HUC.
Click here for an illustrated document that describes HUCs in more detail, as well as how they compare to Wisconsin DNR's Watersheds.
Technical metadata for each of the HUC layers in the Surface Water Data Viewer is available in the links below: