NASQAN National Stream Quality Accounting Network
After a major redesign in 1995, the NASQAN program focused on monitoring the water quality of the nation's largest rivers--the Mississippi (including the Missouri and Ohio), the Columbia, the Colorado, and the Rio Grande. During this phase of the program, NASQAN operated a network of approximately 41 stations where the concentration of an expanded range of chemicals, including pesticides and trace elements, were measured in tandem with stream discharge. Stations were chosen at major nodes within the river basin network to provide characterization of large subbasins of these rivers. (Station Selection Criteria)
The sampling strategy was changed to focus on characterizing the variations in chemical and sediment concentrations that occur during a year, particularly the variation that occurs between low and high flows and during different seasons. In this way, NASQAN data can be used to evaluate mass fluxes or loads of constituents to ultimately determine regional source areas for these materials.