The Upper Colorado River Basin in Colorado and Utah has a drainage area of about 17,800 mi², all except 100 mi² of this area is in Colorado (Driver, 1994). The Colorado River and its tributaries originate in the mountains of central Colorado and flow about 230 mi southwest into Utah. (Map of Upper Colorado River Basin). The Continental Divide marks the eastern and southern boundary of the basin, with altitudes over 14,000 ft. Topography in the western part of the basin generally consists of high plateaus bordered by steep cliffs along the valleys, and the lowest altitude (4,300 ft) is near the Colorado-Utah border. The basin is divided almost equally into two physiographic provinces: the Southern Rocky Mountains in the eastern part and the Colorado Plateau in the western part (Hunt, 1974). The geology varies substantially and consists of crystalline rocks of Precambrian age, stratified sedimentary rocks, and alluvial deposits.

Because of large changes in altitude, the climate (see map) in the basin varies substantially from alpine conditions in the east to semiarid in the west. Mean annual temperatures range from as low as 32.8° F in Gunnison County near the Continental Divide to as high as 54.1° F near Grand Junction, Colo. (Benci and McKee, 1977). Precipitation (see map) in the basin ranges from more than 40 in./yr in the eastern mountainous regions to less than 10 in./yr in the lower altitude western regions. Mountain areas receive most of their precipitation during the winter months when accumulation of snow can exceed on average 100 in. per season. The Grand Junction area receives the largest amount of precipitation during the month of August as a result of weather patterns that produce late-afternoon thunderstorms (Chaney and others, 1987).

The 1990 population (see map) in the largely rural Upper Colorado River Basin was approximately 234,000, less than 10 percent of the total population of the State of Colorado (Bureau of Census, 1990). The largest population center is Grand Junction (30,209), which is located at the confluence of the Colorado and Gunnison Rivers. The larger cities in the basin are located predominantly near agricultural lands or in mountain recreational communities. Population of many counties in the basin increased about 5 percent from 1990 to 1992. Eagle and Summit Counties had the largest increases, of about 12 and 13 percent, respectively.

Land designated for use as either rangeland or forest is the largest land use, (see map) accounting for about 85 percent of the basin. Large parts of the study unit are set aside for recreational use, including all or parts of 4 National Park Service areas, 5 National Forests and numerous wilderness areas, 11 State parks, numerous State Wildlife Management areas, and 17 ski areas. Tourism and recreational activities are a major industry in the study unit past and present. Mining activities past and present are an important land use and have included the extraction of metals and energy fuels. Urban land is one of the smaller land uses in the mostly rural basin. Agriculture is the traditional land use in the study unit in counties such as Delta, Garfield, Gunnison, Mesa, and Montrose. Little crop production is possible without irrigation because of the semiarid climate.

Water-Quality Issues in the Upper Colorado River Basin

Water-quality conditions in the Upper Colorado River Basin are a result of both natural and human factors. Water-quality issues have been summarized by the Colorado Department of Health (1992). The Alva B. Adams Tunnel, which diverts the largest amount of water in the basin, is the probable cause of increases in dissolved-solids concentrations in the Colorado River near Glenwood Springs (Liebermann and others, 1989). Streams have been affected by point-source mine discharge and nonpoint-source runoff from mined areas (Wentz, 1974). Concentrations of cadmium, copper, iron, lead, manganese, mercury, molybdenum, and zinc exceed State water-quality standards for localized reaches of these streams. Coal mining in the area can affect water quality by increases in dissolved solids, particularly sulfate, and increases in trace-element concentrations. Uranium mining was once active in the basin, and mining and milling wastes pose serious threats to ground water from radionuclide contamination. High radium concentrations occur in shallow aquifers in Montrose County in association with uranium mining and milling operations. Many streams in the basin tend to have higher pH values than in other basins in the State, therefore strict un-ionized ammonia standards have been required of wastewater facilities in order to protect cold-water aquatic life. In the mountain areas, communities and resorts generally are located in narrow valleys containing highly permeable gravelly sediments, which have caused some localized shallow ground-water nitrate contamination. High dissolved-solids concentrations are found in irrigated areas near the lower Gunnison and lower Colorado Rivers and their tributaries. Irrigation-return flows as seepage from canals and reservoirs and from field irrigation are the largest human source for dissolved-solids concentrations in the basin. Phosphorus is at low levels throughout the basin. Nitrate generally is at low levels in the basin but is at higher levels in the Roan Creek, Uncompahgre River, and lower Colorado River reaches. Pesticide data are limited for surface water and ground water. Selenium concentrations in the Gunnison and Uncompahgre Rivers exceed U.S. Environmental Protection Agency criteria for protection of aquatic life, and the concentrations may be of concern for fish and water fowl (Butler and others, 1991).

Additional information on the physical setting of the Upper Colorado River Basin and water-quality issues in the basin is detailed in Apodaca and others (1996).

References

Apodaca, L.E., Driver, N.E., Stephens, V.C., and Spahr, N.E., 1996, Environmental Setting and Implications on Water Quality, Upper Colorado River Basin, Colorado and Utah: U.S. Geological Survey Water-Resources Investigations Report 95-4263, 33 p.

Driver, N.E., 1994, National Water-Quality Assessment Program—Upper Colorado River Basin [water fact sheet]: U.S. Geological Survey Open-File Report 94-102, 2 p.

Benci, J.F., and McKee, T.B., 1977, Colorado monthly temperature and precipitation summary for period 1951-1970: Fort Collins, Colorado State University, Climatology Report 77-1, 300 p.

Bureau of Census, 1990, 1992, 1990-1992 census of population and housing: Washington, D.C., data on CD-ROM.

Butler, D.L., Krueger, R.P., Osmundson, B.C., Thompson, A.L., and McCall, S.K., 1991, Reconnaissance investigation of water quality, bottom sediment, and biota associated with irrigation drainage in the Gunnison and Uncompahgre River Basins and at Sweitzer Lake, west-central Colorado, 1988–89: U.S. Geological Survey Water-Resources Investigations Report 91-4103, 99 p.

Chaney, T.H., Kuhn, Gerhard, Brooks, Tom, and others, 1987, Hydrology of Area 58, northern Great Plains and Rocky Mountain coal provinces, Colorado and Utah: U.S. Geological Survey Water-Resources Investigations Open-File Report 85-479, 103 p.

Colorado Department of Health, 1992, Water quality in Colorado 1992: Denver, Colorado Water Quality Control Division, 103 p.

Hunt, C.B., 1974, Natural regions of the United States and Canada: San Francisco, W.H. Freeman and Company, 725 p.

Liebermann, T.D., Mueller, D.K., Kircher, J.E., and Choquette, A.F., 1989, Characteristics and trends of streamflow and dissolved solids in the Upper Colorado River Basin, Arizona, Colorado, New Mexico, Utah, and Wyoming: U.S. Geological Survey Water-Supply Paper 2358, 64 p.

Wentz, D.A., 1974, Effect of mine drainage on the quality of streams in Colorado, 1971–72: Water Conservation Board, Water Resources Circular 21, 117 p.