Hydrologic Unit Codes
handout exercisesuggested grade levels: 9-12

view Idaho achievement standards for this lesson

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. 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. Alaska is region 19, the Hawaii Islands constitute 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 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." A watershed is the area of land that catches rain and snow and drains or seeps into a marsh, stream, river, lake or groundwater. Homes, farms, ranches, forests, small towns, big cities and more can make up watersheds. Some cross county, state, and even international borders. Watersheds come in all shapes and sizes. Some are millions of square miles, others are just a few acres. Just as creeks drain into rivers, watersheds are nearly always part of a larger watershed. An eight-digit code 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) 01080204: 01 - the region 0108 - the sub-region 010802 - the accounting unit 01080204 - the cataloging unit An 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. Click here to look at the HUC catalog for regions 16 and 17, which encompass Idaho. A series of uniform, nationally consistent State Hydrologic Unit Maps that accurately delineate the hydrographic boundaries of major U.S. river basins has been prepared by the U.S. Geological Survey in cooperation with the U.S. Water Resources Council. These maps provide a standardized base for use by Federal and State water resources agencies throughout the country. The project, initiated in 1972 by the U.S. Geological Survey's Office of Water Data Coordination and supported by the Survey's Resource Planning Analysis Office, has resulted in a standard geographical framework for more detailed water- and related land-resources planning. Prior to publication of these maps, water-resources planners had been using a variety of criteria for naming and coding drainage basins and for delineating hydrologic boundaries. With the publication of the hydrologic unit maps, water-resources managers now have a consistent starting point for planning, as well as an aid for organizing and disseminating data. 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. Using the U.S. Geological Survey State map series as a base, they delineate river basins in the United States that have drainage areas greater than 700 square miles. The four-color maps show a distinct numeric code assigned to each river basin and provide information on drainage, culture, hydrography, and hydrologic boundaries for each of the 21 regions and 222 subregions designated by the Water Resources Council. They also depict the boundaries and codes of 352 accounting units within the National Water Data Network and approximately 2,100 cataloging units of the Geological Survey's Catalog of Information on Water Data. The hydrologic units are arranged within each other, starting from smallest (Cataloging Units) to the largest (Regions). Also included on the maps are State and County codes that use the Federal Information Processing Standards (FIPS) The uses of the State Hydrologic Unit Maps are many, From the standpoint of data collection, storage, and manipulation, a standard coding system is necessary for those wishing to use all data from a particular river basin. 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 (including the Forest Service, Soil Conservation Service, Fish and Wildlife Service, National Park Service, Council on Environmental Quality, National Weather Service, and Water Resources Council) and State, county, and local agencies are using the hydrologic units for codifying and displaying the data that were collected locally and nationwide. During Phase 2 of the Digital Atlas this module will be expanded to include information and figures on Idaho HUCs, drainages and watersheds, courtesy of the state's Division of Environmental Quality. Additional material coming for phase 2 will include a discussion of physical geography and the development of drainages. Chapter 2 of Rocks, Rails & Trails offers useful material and maps on this subject covering southeastern Idaho. Look at some pictures: * Southeast Idaho LANDSAT * Snake River Plain Hillshade * Thematic Map 1 - Columbia River/Snake River * Thematic Map 2 - Snake River Plain * Thematic Map 3 - Eastern Snake River Plain

Related Lesson Topics:
Geology: Geology Topics

Lesson Plan provided by Vita Taube, 2000
Idaho Achievement Standards (as of 7/2001) met by completing this activity: