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Jqq4 I)) ~re{!ft;ji CUI '- ',' !/}., ,)/-{f [, cAf 9' ;t3 J WATER FACT SHEET u.s. GEOLOGICAL SURVE~ DEPARTMENT OF THE INTERIOR NATIONAL WATER-QUALITY ASSESSMENT PROGRAM-uPPER COLORADO RIVER BASIN In 1991, the U.S. Geological Survey, U.S. Department of the Interior, began a National Water-Quality Assessment (NAWQA) program. The long-term goals of the NAWQA pro- gram are to describe the status and trends in the quality of a large, representative part of the Nation's surface- and ground- water resources and to identify the major natural and human factors that affect the quality of these resources. In meeting these goals, the program will produce a wealth of water-quality information that will be useful to policy makers and managers at the National, State, and local levels. The NAWQA program emphasis is on regional-scale water-quality problems. The program will not diminish the need for subregional and site-specific studies and monitoring presently designed and conducted by Federal, State, and local agencies to meet their individual needs. The NAWQA pro- gram, however, will provide a regional framework for conduct- ing many of these activities and an understanding about regional and national water quality that cannot be acquired from individual, site-specific programs and studies. Studies of 60 hydrologic systems that include parts of most major river basins and aquifer systems (study-area investiga- tions) are the building blocks of the national assessment. The 60 study areas range in size from 1,000 square miles to more than 60,000 square miles and represent 60 to 70 percent of the Nation's water use and population served by public-water sup- plies. Twenty study-area investigations were started in 1991, 20 additional started in 1994, and 20 more are planned to start in 1997. The Upper Colorado River Basin in Colorado and Utah was selected as one of 20 study areas that began assess- ment activities in 1994. DESCRIPTION OF THE UPPER COLORADO RIVER BASIN More than 99 percent of the Upper Colorado River Basin study area is in Colorado; the remaining is in Utah. It has a drainage area of about 17,800 square miles. The primary river within the basin, the Colorado River, originates in the moun- tains of central Colorado and flows about 230 miles southwest into Utah. The headwaters of the Colorado River and most of its tributaries originate in the mountains that form the eastern and southern boundaries of the study area. This boundary is the Continental Divide. The major tributaries to the Colorado River in the study area are the Blue, Eagle, Roaring Fork, Gun- nison, and Uncompahgre Rivers. This two-State study area is predominantly rural and is inhabited by about 234,000 people, more than 99 percent resid- ing in Colorado. The majority of the basin population is con- centrated in the Grand Junction, Colorado area. Tourism is a major year-round industry and accounts for substantial increases in population primarily during winter and summer. Physiography, Geology, and Hydrology The study area is almost equally divided between the Southern Rocky Mountain province to the east and the Colo- rado Plateau province to the west. The area's rugged landscape displays north-northwest-trending mountain ranges in the east. The mountains are flanked by steeply dipping sedimentary rocks. To the west are high plateaus bordered by steep cliffs along the valleys. Distinctive mesas are conspicuous features in the area. The topography varies greatly, and altitudes range from more than 14,000 feet along the Continental Divide to about 4,300 feet near the Colorado-Utah State line. The pre- dominant vegetation is pine, fir, and spruce forests in the moun- tains and greasewood and sagebrush in the lower altitudes. The Upper Colorado River Basin is underlain by rocks ranging in age from Precambrian (1,800 million years) to Qua- ternary. The long history of sedimentation, erosion, and tec- tonic activity associated with the formation of these rocks has resulted in a complex geologic setting. Precambrian forma- tions of gneisses, schists, and granites form the core of the high mountains and are very resistant to erosion. Sedimentary rocks predominate through the rest of the study area and consist pri- marily of sandstone, siltstone, and shale and local occurrences of evaporites. Several widespread geologic formations were deposited in marine environments and resulted in bedded and disseminated sodium chloride (halite) and calcium sulfate (gypsum), as well as clay that has high concentrations of exchangeable sodium and magnesium. These formations con- tribute substantially to the natural sources of salinity in the study area. Sedimentary formations also contain deposits of oil, oil shale, coal, and natural gas. The alluvium consists of unconsolidated deposits that typically transmit the most ground water in the study area. Structural features, including anti- clines, domes, and faults, expose large sequences of strata in the study area. Streamflow has marked seasonal and annual variability. Most annual streamflow in the Upper Colorado River Basin results from snowmelt during spring and early summer. Most annual floods on these streams occur during the snowmelt period. Although the magnitude of these floods can be quite large, exceptionally large snowmelt floods that could cause severe flooding are very uncommon. Low flows on perennial streams are sustained primarily by flows from ground water. Gradual melting of perpetual snowfields and reservoir releases also augment low flows on some streams. Thunderstorms in EXPlANATION wa1er bodiE'9 streams _ .., sludy unit boundary 40. locaHon of study area I~ L:r!aj o 15 30 MILES 38' o 20 40 KILOMETERS