The United States is divided and sub-divided into successively smaller hydrologic units which are classified into four levels: regions, sub-regions, accounting units, and cataloging units. The hydrologic units are arranged within each other, from the smallest (cataloging units) to the largest (regions). Each hydrologic unit is identified by a unique hydrologic unit code (HUC) consisting of two to eight digits based on the four levels of classification in the hydrologic unit system. Cataloging units are also called watersheds.
Data are available at the 1:2,000,000 scale or the 1:250,000 scale over the internet. The codes for the units are the same for both scales of data. The data sources below offer data in several formats. The tools are directed towards the E00 format data, and using that data in ArcView.
Once the data has been download from the Internet to a PC and imported into ArcView, you will have a number of files. Depending on the import routine you have run (which is dependent on your operating system and software version) you may or may not have the four files available from the second download option below. These are simple DBF files each having two columns - an huc code of 2, 4, 6, or 8 digits (huc2, huc4, huc6, and huc), and a description. For example, n2.dbf has two columns - huc2 and description.
What you will not have is the table available from the first download option below - a DBF file with all the huc codes that can be used to relate between the descriptions and the attribute file that comes with the data. This file, allhuc.dbf, contains the columns huc2, huc4, huc6, and huc. Using this file you can relate any of the above files with descriptions in them to the actual data. You will still need to link various tables (in this case dbf files), depending on which level of hydrologic units you want to display.
Hydrologic Unit Data Sources
USGS Hydrologic Unit data at
1:250,000 and 1:2,000,000 scales. The 1:250,000 data is available by regions or for the
entire country.
NRCS Hydrologic Unit
information on the 1:24,000 scale data. This site has pointers to the process
for updating the data, contacts for obtaining the data, and a status map.
Tools for Using Hydrologic Unit Data
Hydrologic data attribute table
in a downloadable Dbase III format. This table (allhuc.dbf) combines the 2, 4, 6, and 8
digit codes into one table that can be used to look up the descriptions from the other
tables listed below as well as relate to the attribute table that is associated with the
data. The file is compressed using PKZip. Depending on your browser you may need to right
click to download.
2, 4, 6, and 8 digit hydrologic
unit codes and descriptions. There are four files (n2.dbf, n4.dbf, n6.dbf, and n8.dbf),
all compressed into one zipped file using PKZip. Depending on your browser you may need to
right click to download.
For additional information regarding this Web page, contact Deb Southworth Green , in the Division of Information Resources and Technology Management, at Deb_Green@fws.gov
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Keywords=GIS, data, watersheds, huc, hydrologic, units
Last Modified January 03, 2001 10:34 AM