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The Hydrologic Unit Code (HUC) is a hierarchical land area classification system created by the United States Geological Survey (USGS). It is based on the surface hydrologic features in a standard geographic framework. It is a multi-tiered system, where each level adds two digits to the code in a broad to narrow approach of defining watersheds.

Iowa has two HUC regions - 07 Mississippi River and 10 Missouri River. Within the state, the two most commonly used HUC tiers are the HUC8 (Sub-basin) and HUC12 (Sub-watershed).

More information on Hydrologic Unit Codes

USGS Hydrologic Unit