In 1991, the U.S. Geological Survey (USGS) began to implement the National Water-Quality Assessment (NAWQA) program. Goals of the NAWQA program are to (1) describe water-quality conditions for a large part of the Nation’s freshwater streams, rivers, and aquifers; (2) describe how water quality is changing over time; and (3) improve the understanding of the primary natural and human factors that affect water-quality conditions (Leahy and others, 1990). To complete this program, the USGS plans to investigate 60 river basins and aquifer systems (study units) throughout the United States. Information obtained from these different study units will help Federal, State, and local agencies make needed management, regulatory, and monitoring decisions to better protect, use, and enhance water resources. This fact sheet, based on Apodaca and others (1996), describes how the environmental setting of the Upper Colorado River Basin (UCOL) study unit affects water quality in the basin. Information about the environmental setting, which is based on natural and human factors, was used to design surface-waterand ground-water-quality sampling networks for the study unit. The UCOL study unit, located in western Colorado, has a drainage area of about 17,800 square miles (fig. 1). The primary river within the basin, the Colorado River, originates in the mountains of central Colorado and flows about 230 miles southwest into Utah. Major tributaries to the Colorado River are the Blue, Eagle, Roaring Fork, and Gunnison Rivers. In 1990, population in the basin was about 234,000 (Bureau of Census, 1990). The largest population center in the basin is the area around Grand Junction, Colo.