Hydrologic Unit: Watersheds in the United States and the Caribbean were delineated by the U.S. Geological Survey using a national standard hierarchical system based on surface hydrologic features and are classified into four types of hydrologic units: first-field (region), second-field (sub-region), third-field (accounting unit), and fourth-field (cataloguing unit). A fifth-field of classification (watershed) and sixth-field (sub-watershed) are currently under development. The United States and Caribbean are divided into 21 regions, 221 subregions, 378 accounting units, and 2,264 cataloguing units. A hydrologic unit can accept surface water directly from upstream drainage areas, and indirectly from associated surface areas such as remnant, non-contributing, and diversions to form a drainage area with single or multiple outlet points. Hydrologic units are only synonymous with classic watersheds when their boundaries include all the source area contributing surface water to a single defined outlet point. Hydrologic Unit Code: Each hydrologic unit is identified by a unique hydrologic unit code (HUC) consisting of two to twelve digits based on the six levels of classification: 8-digit HUC (Cataloguing Unit) 2-digit HUC first-level (region) 4-digit HUC second-level (subregion) 6-digit HUC third-level (accounting unit) 8-digit HUC fourth-level (cataloguing unit) 10-digit HUC fifth-level (watershed) 12-digit HUC sixth-level (subwatershed). This layer represents the 12-digit HUC sixth-level (subwatershed).

Vegetation characterization for Agate Fossil Beds National Monument, NE

WMS

Vegetation characterization for Badlands National Park, SD

Vegetation characterization for Congaree National Park, SC

Vegetation characterization for Allegheny Portage Railroad, PA