The upper Colorado River; National Water-Quality Assessment Program; surface-water-monitoring network

The U.S. Geological Survey began full implementation of the National Water-Quality Assessment (NAWQA) program in 1991. The long-term goals of the NAWQA program are to (1) describe current 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 understanding of the primary natural and human factors that affect water-quality conditions (Leahy and others, 1990). To meet these goals, 60 study units representing the Nation's most important river basins and aquifers are being investigated. The program design balances the unique assessment requirements of individual study units with a nationally consistent design structure that incorporates a multiscale, interdisciplinary approach for assessment of surface and ground water. The Upper Colorado River Basin (UCOL) is one of the 60 NAWQA study units; hydrologic and water-quality assessments of the UCOL began in 1994. The study unit has a drainage area of about 17,800 mi2, and the primary river within the basin, the Colorado River, originates in the mountains of central colorado and flows about 230 mi southwest into Utah. Major tributaries to the Colorado River in the study unit are the Blue, Eagle, Roaring Fork, and Gunnison Rivers. The Colorado River is the major supply of water to the southwestern United States. Streamflow from the study unit accounts for about 40 percent of the streamflow of the Colorado River at Lees Ferry. The UCOL study unit is divided almost equally into two physiographic provinces-the southern Rocky Mountains in the eastern part and the Colorado Plateau in the western part. The Southern Rocky Mountain province is characterized by northnorthwest- trending mountains of crystalline rocks that range in elevation from 11,000 to more than 14,000 ft. The Colorado Plateau province consists of high plateaus of sedimentary rocks with elevations ranging from about 5,500 to 8,500 ft. Land use, classified as rangeland or forest, accounts for about 85 percent of the basin. The other major land uses in the basin are agriculture, mining, and urban. Surface water used for irrigation accounts for about 97 percent of the total offstream water use. A surface-water-monitoring network for the UCOL study unit was designed considering the critical natural and human factors that affect surfacewater quality in the basin.