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r {qq(p U.S f:1.eofo leal Surv <1;1/ 't US GS Upper Colorado River- Basi~ NAWQA F ACT SHE E T What Affects Water Quality In the Upper Colorado River Basin? -Lori E. Apodaca, Vcrlin C. Stephens, and Nancy E. Driver The Upper Colorado River Basin is 1 of 60 study units selected for water-quality assessment by the U.S. Geological Survey as part of the National Water-Quality Assessment program. Understanding the environmental setting of the Upper Colorado River Basin study unit is important in evaluating water-quality issues in the basin. Information about the environmental setting identifies the basin characteristics and includes natural and human factors that affect the physical, chemical, and biological quality of water in the basin. This information has been used to design surface-water- and ground-water-quality sampling networks in the basin to assess overall water quality of the basin. INTRODUCTION In 1991, the U.S. Geological Survey (USGS) began to imple- ment the National Water-Quality Assessment (NAWQA) program. Goals of the NA WQA 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 environ- mental 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-water- and ground-water-quality sampling networks for the study unit. The UCOL study unit, located in western Colorado, has a drain- age area of about 17,800 square miles (fig. I). 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, Roar- ing Fork, and Gunnison Rivers. In 1990, population in the basin was about 234,000 (Bureau of Census, 1990). The largest popula- tion center in the basin is the area around Grand Junction, Colo. ENVIRONMENTAL SETTING The environmental setting of the UCOL study unit provides a framework for understanding the basin characteristics, which are important in the design of an integrated study of the surface water and ground water of the basin. Natural factors that affect background water-quality conditions in the UCOL basin are physiography, climate, geology, and soils. Human factors that affect water-quality conditions are water management and use and land use in the basin. UPPER COLORADO RIVER BASIN STUDY UNIT UTAH 00 COLORA~7~ W EXPLANATION ..... Physiographic Province Boundary Study Unit Boundary Surface-Water-Sampling Site 20 40 MILES I I 20 40 KILOMETERS Figure 1. Location of the Upper Colorado River Basin study unit.