Skip to main content
U.S. flag

An official website of the United States government

Hydrologic Unit Maps

May 10, 2021

The U.S. is sub-divided into successively smaller hydrologic units which are classified into four levels: regions, sub-regions, accounting units, and cataloging units. Each unit is identified by a unique hydrologic unit code (HUC) consisting of two to eight digits based on its classification. The Hydrologic Unit Maps site provides information and data for current and historical hydrologic units, names, and numbers.

Regional Hydrologic Unit Map
Screenshot of Hydrologic Unit Maps tool.