Hydrologic Unit Maps

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.

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