A digital representation of the watersheds of 43 sites on large river systems sampled by the National Stream Quality Accounting Network (NASQAN) of the U. S. Geological Survey (USGS) in 2000 was created primarily from 1:250,000 hydrologic units(HUCs) in the United States. Watershed information from Canada and Mexico was incorporated to complete the areas draining to the sampling sites from outside the United States. The sampled rivers are in one of four major river systems: the Mississippi, the Colorado, the Rio Grande, or the Columbia.
Additional publication details
USGS Unnumbered Series
Watersheds for U.S Geological Survey National Stream Quality Accounting Network (NASQAN) sampling sites 1996-2000