Hydrologic Unit Code (HUC) boundaries are a subdivision of the United States made by the USGS, to show major and minor river basins. Each river basin has a numeric code, the major basins have 2 digit HUC boundary code, while the smaller, sub-basins have 4, 6, and 8 digit codes. For example, a large river basin zone may have a HUC2 code of 12. The smaller basins, locaed within that zone would have HUCs equal to 1201, 1202, etc. The sub-basins can be even further subdivided by, using HUC6 and HUC8.
Boundaries and numeric codes are depicted for 21 regions, 222 subregions, 352 accounting units, and 2,100 cataloging units. River basins are delineated that have drainage area greater than 700 square miles. Also included on the maps are State and county codes that use the Federal Information Processing Standards (FIPS). State maps are published at a scale of 1:500,000; the U.S. map (unfortunately, out of print) is at a scale of 1:2,500,500.
Digital data sets for hydrologic units are available at scales of 1:2,000,000 and 1:250,000. Each is a single coverage for the conterminous United States. Attributes of the 1:2,000,000-scale version include basin names.
Go to HUC Web Site .
Go to Obtaining Hydrologic Information through the Internet .