The 2-Digit Hydrologic Units, a component of the Watershed Boundary Dataset (WBD), depict the major water resource regions of the United States. A water resource region is the first level of classification used by the United States Geological Survey to … Continued
The 4-Digit Hydrologic Units, a component of the Watershed Boundary Dataset (WBD), depict the second level hydrologic subdivision of the United States. A 4-digit hydrologic unit includes the area drained by a river system, a reach of a river and … Continued
The 6-Digit Hydrologic Units, a component of the Watershed Boundary Dataset (WBD), depict the third level hydrologic subdivision of the United States. A 6-digit hydrologic unit may subdivide a 4-digit hydrologic unit or may be equivalent. There are 401 6-digit … Continued