The U.S. Geological Survey began its National Water-Quality Assessment (NAWQA) Program in 1991, systematically collecting chemical, biological, and physical water-quality data from study units (basins) across the Nation. In 1999, the NAWQA Program developed a data warehouse to better facilitate national and regional analysis of data from 36 study units started in 1991 and 1994. Data from 15 study units started in 1997 were added to the warehouse in 2001. The warehouse currently contains and links the following data:

• Chemical concentrations in water, sediment, and aquatic-organism tissues and related quality-control data from the USGS National Water Information System (NWIS),
• Biological data for stream-habitat and ecological-community data on fish, algae, and benthic invertebrates,
• Site, well, and basin information associated with thousands of descriptive variables derived from spatial analysis, like land use, soil, and population density, and
• Daily streamflow and temperature information from NWIS for selected sampling sites.