Hydrological code
United States
Columns: Name | Level | Digits | Average size (square miles) | Number of HUs (approximate) | Example name | Example code (HUC)
Region | 1 | 2 | 177,560 | 21 | Pacific Northwest | 17 | Subregion | 2 | 4 | 16,800 | 222 | Lower Snake | 1706 | Basin | 3 | 6 | 10,596 | 370 | Lower Snake | 170601 | Subbasin | 4 | 8 | 700 | 2200 | Imnaha River | 17060102 | Watershed | 5 | 10 | 227 (40,000–250,000 acres) | 22000 | Upper Imnaha River | 1706010201 | Subwatershed | 6 | 12 | 40 (10,000–40,000 acres) | 160000 | South Fork Imnaha River | 170601020101