1:250,000-scale Hydrologic Units of the United States
The Geographic Information Retrieval and Analysis System (GIRAS)
was developed in the mid 70s to put into digital form a number
of data layers which were of interest to the USGS. One of these
data layers was the Hydrologic Units. The map is based on the
Hydrologic Unit Maps published by the U.S. Geological Survey …