In 1995, the U.S. Geological Survey's (USGS) National Stream Quality Accounting Network (NASQAN) Program began monitoring the water quality of the Columbia River Basin, applying a basinwide approach in order to understand water quality on a regional scale. A primary objective of the Columbia NASQAN Program is to provide an ongoing characterization of the concentrations and mass flux (amount of material or load passing a location per unit time, generally expressed as tons per day) of sediment and chemicals at key locations in the basin. These data can then be used to determine regional source areas for these materials, and to assess the effect of human influences on observed concentrations and constituent loads. NASQAN complements the ongoing USGS National Water Quality Assessment (NAWQA) Program, which is performing a detailed assessment in three subbasins of the Columbia River Basin. NASQAN monitors the larger rivers in the basin, downstream of NAWQA study units.