NAWQA Upper Colorado River Basin Study Completed

Physical Setting of the Upper Colorado River Basin

The Upper Colorado River Basin in Colorado and Utah has a drainage area of about 17,800 square miles, 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.   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.

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Because of large changes in altitude, the climate 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 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 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, 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

The NAWQA Program was not intended to assess the quality of the Nation’s drinking water, such as by monitoring water from household taps. Rather, the assessments focus on the quality of the resource itself, thereby complementing many ongoing Federal, State, and local drinking-water monitoring programs. The comparisons made in this study to drinking-water standards and guidelines are only in the context of the available untreated resource.

Water-quality conditions in the Upper Colorado River Basin are a result of both natural and human factors.

• 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 groundwater 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 groundwater.
• 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).

USGS Circular 1214 summarizes major findings about water quality in the Upper Colorado River Basin that emerged from an assessment conducted between 1996 and 1998.