A national standard system of watersheds that are classified into four types of units: regions, sub-regions, accounting units, and cataloging units. The hydrologic units are arranged within each other, from the smallest (cataloging units or sub-basin) 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. A standardized fifth-level of classification or 10-digit hydrologic unit (watershed) has recently been developed. Locally, a non-standard sixth-level sub-watershed also may have been developed.

A distinct watershed or river basin defined by an 8-digit code.

all land and water within a drainage area.

is a geographic area representing part or all of a surface drainage basin or distinct hydrologic feature as delineated by the Office of Water Data Coordination on State Hydrologic Unit Maps; each hydrologic unit is identified by an eight-digit number.