Hydrologic Units (Classification Codes) USGS A means by which the Unites States has been divided and subdivided into successively smaller Hydrologic Units which have been classified into four levels consisting of 21 major water resources regions, 222 subregions, 352 accounting units and 2,149 cataloging units. The first level of this U.S. Geological Survey (USGS) classification system divides the U.S. into 21 major geographic areas, or regions. These geographic areas (hydrologic areas based on surface topography) contain either the drainage area of a major river or the combined drainage areas of a series of rivers. Eighteen of the regions occupy the land area of the conterminous U.S., Alaska is region 19, the Hawaiian Islands constitute region 20, and Puerto Rico and other outlying Caribbean areas are region 21. (The Pacific Trust Territories are a potential region 22.) 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 accounting units are used by the USGS for designing and managing the National Water Data Network. The fourth level of classification is the cataloging unit, the smallest element in the hierarchy of hydrologic units. A cataloging unit is a geographic area representing part 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 (2,149 in the U.S.) that are used by the USGS for cataloging and indexing water-data acquisition activities in the “Catalog of Information on Water Data.” 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 subregions, the first six digits identify the accounting unit, and the addition of two more digits identify the cataloging unit. See Water Resources Regions United States for a complete listing of the 21 major water resources regions.