Hydrologic Unit Code Explained

Hydrologic Unit Codes (HUC)are part of a United States Geologic Survey (USGS) watershed classification system based on size. In this system the United States is divided into four levels of classification:

• First level divides the nation into 21 major geographic regions. 2-digit HUC
• Second level divides those 21 regions into 221 sub-regions. 4-digit HUC
• Third level divides the sub-regions into 378 accounting units. 6-digit HUC
• Fourth level divides the accounting units into 2264 cataloging (watershed) units. 8-digit HUC

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