Classification of Hydrologic Drainage Basins
              in the United States

    Hydrologic Units, Hydrologic Unit Codes, and
              Hydrologic Unit Names

                 (Modified from Slack and Landwehr, 1992 and
                       Seaber, Kapinos, & Knapp, 1987)


                               Hydrologic Units

         The United States was divided and sub-divided into successively
      smaller hydrologic units which were 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 area, or regions.  These geographic areas
      (hydrologic areas based on surface topography) 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 sub-regions.  A sub-region 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 sub-
      regions into accounting units.  These 352 hydrologic accounting
      units nest within, or are equivalent to, the sub-regions.

         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 basins, or a distinct
      hydrologic feature.  These units subdivide the sub-regions and
      accounting units into approximately 2150 smaller areas.


                            Hydrologic Units Codes

         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.


                            Hydrologic Unit Names

         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.

         A complete list of all hydrologic unit codes, their names, the
      names of the States or outlying areas in which they reside, and
      their drainage areas is given in the file HUC_NAME.TXT.  A
      condensed version of that file, giving the drainage areas
      of the regions and sub-regions only, is provided in HUC_AREA.DAT.