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WHAT IS A HYDROLOGIC UNIT CODE?
A watershed address consists of a name and a number (for example, St. Mary's River watershed, 04100004). The 8-digit number is an example of a Hydrologic Unit Code or HUC. The Hydrologic Unit system is a standardized watershed classification system developed by USGS in the mid 1970s, and since revised in 1995. Hydrologic units are watershed boundaries organized in a nested hierarchy by size.
The United States is divided and sub-divided into successively smaller hydrologic units which are classified into six levels: regions, sub-regions, basins, subbasins, watersheds and subwatersheds. The hydrologic units are arranged within each other, from the smallest (subwatersheds) to the largest (regions). Each hydrologic unit is identified by a unique hydrologic unit code (HUC) consisting of two to fourteen digits based on the six levels of classification in the hydrologic unit system. The table below shows a summary of the classifications.
No. of Digits |
Classification |
Unit Size |
2 |
Region |
177,560 square miles (average) |
4 |
Subregion |
16,800 square miles (average) |
6 |
Basin |
10, 596 square miles (average) |
8 |
Subbasin |
703 square miles (average) |
11 |
Watershed |
40,000 - 250,000 acres (range) |
14 |
Subwatershed |
10,000 - 40,000 acres (range) |
Other Information
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