Hydrologic Unit Codes

Drainage basins in the United States have been divided and sub-divided at four different levels and each assigned a unique hydrologic unit code (HUC) consisting of eight digits based on these four levels. The four levels from largest to smallest are regions, sub-regions, accounting units, and cataloging units.

The first level of classification divides the nation into 21 major geographic areas. 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. Alaska is region 19, the Hawaii Islands region 20, and Puerto Rico and other outlying Caribbean areas are region 21.

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 or 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 are the equivalent of a drainage basin or "watersheds".

An eight-digit code (17040206) uniquely identifies each of the four levels of classification within four two-digit fields. The first two digits identify the water-resources region; the first four digits identify the sub-region; the first six digits identify the accounting unit, and the addition of two more digits for the cataloging unit completes the eight-digit code. An example is given here using hydrologic unit code (HUC) 17040206 for the American Falls area. A double 00 in the two-digit accounting unit field indicates that the accounting unit and the sub-region are the same. Likewise, if the cataloging unit field is 00, it is the same as the accounting unit.

In addition to hydrologic unit codes, each hydrologic unit has been assigned a name corresponding to the principal hydrologic feature(s) within the unit. In the absence of such features, the assigned name may reflect a cultural or political feature within the unit. All regions and sub-regions are uniquely named; however, the accounting units are uniquely named only within each region, and the cataloging units are uniquely named only within each accounting unit. Duplication of some names at the cataloging unit level is unavoidable because a large number of streams found throughout the nation share the same names.

Hydrologic unit codes were initiated by the U.S. Geological Survey's Office of Water Data in the fall of 1972 in cooperation with the U.S. Water Resources Council and supported by the U.S. Geological Survey's Resources and Land Information program. Before the HUC system was initiated drainage basins were divided and coded using various methods depending on the organization using them. The HUC codes have created a unifed system between agencies and water-resource planners. These maps, published at a scale of 1:500,000 (1 inch equals nearly 8 miles), present twice the detail of previous river basin maps.

The HUC maps have created a standard for the data collection, storage, and manipulation of files associated with particular river basins.drainage divided. The Geological Survey is using the coding system to document all its water-data collection activities and its data-planning efforts. The Survey's National Water Data Exchange (NAWDEX) system has incorporated the code into its computer system to allow all its members easier access to data holdings that consist of more than a billion water-resource measurements. Other Federal agencies, state county, and city governments are using the HUC's (hydrologic units codes) for coding and managing the data that is collected locally and nationwide.