Hydrologic Unit Codes: What Are They?
National Watershed Boundary Dataset
Hydrologic unit codes, or HUCs, identify all the drainage basins in the United States in a nested arrangement, ranging from the largest (regions) to the smallest (cataloging units).
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. 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.
HUC-2
Regions
The first level of classification divides the United States into 21 regions. Regions are geographic areas that 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.
HUC-4
Sub-regions
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.
HUC-6
Basins
The third level of classification subdivides many of the subregions into basins (formerly called accounting units). Basins are nested within, or can be equivalent to the subregions.
HUC-8
Sub-basins
The fourth level of classification subdivides basins into subbasins (formerly called cataloging unit). The geographic area representing part or all of a surface drainage basin, a combination of drainage basins, or a distinct hydrologic feature. Subbasins were originally the smallest element in the hierarchy, until 2010 when the NRCS began delineating two smaller levels.
HUC-10
Watersheds
The fifth level of classification divides subbasins into watersheds, which are approximately 40,000 to 200,000 acres.
HUC-12
Sub-watersheds
The sixth level of classification divides watersheds into subwatersheds, which are approximately 10,000 to 40,000 acres. Subwatersheds represent the local level by capturing primary tributary systems.
