URL: water.usgs.gov/GIS/huc.html COST: Free for download. Some cost associated with obtaining CDs from USGS. AGENCY: USGS FORMAT: 1:2,000,000 available in SDTS, Shapefile, ArcInfo Export formats PROJECTION: 1:2,000,000 in Decimal Degrees, NAD83; 1:250,000 is Albers Conical Equal Area

DESCRIPTION: The United States is divided and sub-divided into successively smaller hydrologic units that 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.

The first level of classification divides the Nation into 21 major geographic areas, or regions. These geographic areas contain either the drainage area of a major river, such as the Missouri region, or the combined drainage areas of a series of rivers, such as the Texas-Gulf region, which includes a number of rivers draining into the Gulf of Mexico. Eighteen of the regions occupy the land area of the conterminous United States. The second level of classification divides the 21 regions into 222 subregions. A subregion includes the area drained by a river system, a reach of a river and its tributaries in that reach, a closed basin(s), or a group of streams forming a coastal drainage area. The third level of classification subdivides many of the subregions into accounting units. These 352 hydrologic accounting units nest within, or are equivalent to, the subregions. The fourth level of classification is the cataloging unit, the smallest element in the hierarchy of hydrologic units. Efforts are underway to add further levels of subdivisions. A cataloging unit is a geographic area representing part of all of a surface drainage basin, a combination of drainage basins, or a distinct hydrologic feature. These units subdivide the subregions and accounting units into smaller areas. There are 2150 Cataloging Units in the Nation. Cataloging Units sometimes are called "watersheds."

The four classification levels are demonstrated using a watershed and the corresponding information. Notice that the first table (corresponding to huc 1:2,000,000) shows the numbers for HUC 2, 4, and 6 as well as for Region (REG), Sub-region (SUB), Accounting Unit (ACC), and Cataloging Unit (CAT). The same numbers are evident in the second table (corresponding to huc 1:250,000) where the numbers for REGION, SUBREGION, ACCTUNIT, and HYDROUNIT are put together to create the HUC number. The tables demonstrate how the numbers build upon each other to create the 8-digit HUC unit.

There are two HUC products available that differ primarily by the scale of the base map used to delineate and derive the HUC boundaries. Varying base maps means that the HUC boundaries are variable between the two maps. In this graphic, the 1:2,000,000 HUCs are shown in black; the 1:250,000 HUCs are shown in red and the Connecticut Towns layer is shown in light gray. Neither dataset should be used for detailed analysis beyond the scale of the original mapping unit (1:2,000,000 or 1:250,000).

1:250,000-scale Hydrologic Units (huc250k)

The map is based on the Hydrologic Unit Maps published by the U.S. Geological Survey Office of Water Data Coordination, together with the list descriptions and name of region, subregion, accounting units, and cataloging unit.

The data for this coverage was originally collected for the Geographic Information Retrieval and Analysis System (GIRAS) at a scale of 1:250K. Some areas, notably major cities in the west, were recompiled at a scale of 1:100K. The coverage was compiled to provide the National Water Quality Assessment (NAWQA) study units with an intermediate- scale river basin boundary for extracting other GIS data layers.

DOWNLOAD: The data are available for download at water.usgs.gov/lookup/getspatial?huc250k. The entire data set can be downloaded, or it can be downloaded by region. Click on the FTP link to download the data in Arc/INFO Export format (e00.gz) or STDS format (stds.tgz).

METADATA: Metadata in FGDC format is available at water.usgs.gov/GIS/metadata/usgswrd/huc250k.html.

1:2,000,000-scale Hydrologic Units (huc2m)

This file contains hydrologic unit boundaries and codes for the conterminous United States along with Alaska, Hawaii, Puerto Rico and the U.S. Virgin Islands. It was revised for inclusion in the National Atlas of the United States of America, and updated to match the streams file created by the USGS National Mapping Division (NMD) for the National Atlas of the United States of America.

DOWNLOAD: The data are available for download at www.nationalatlas.gov/hucsm.html. Click to download the data in shapefile, SDTS, and ArcInfo Export format.

METADATA: Metadata in FGDC format can be viewed at water.usgs.gov/GIS/metadata/usgswrd/huc2m.html and a Map Layer Description File is available at www.nationalatlas.gov/hucsm.html.

RESULTS: Downloading either HUC dataset results in a watershed layer that can be used with other geospatial data. The 1:250,000 HUC data is available for download by region, as displayed in the first graphic below. The second graphic shows the cataloging units for the state of Connecticut.

The four levels of HUCs are shown below for the continental U.S. to demonstrate the difference in size between levels.

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