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and birds. The Uncompahgre Project area is a major
<br />source of selenium to the Gunnison River, and the
<br />effects of selenium on the endangered fish and birds
<br />in this area are not known (Butler and others, 1991).
<br />The presence of other trace elements in the water
<br />supply can result from irrigation. This is due to reuse
<br />of irrigation water and leaching of naturally occurring
<br />trace elements from the soils.
<br />SUMMARY
<br />The goals of the U.S. Geological Survey's
<br />NAWQA program are to describe current conditions
<br />for a large part of the Nation's surface- and ground-
<br />waterresources, describe how water quality is
<br />changing over time, and identify the major natural and
<br />human factors that affect the water quality. The Upper
<br />Colorado River Basin study is 1 of 60 study units
<br />selected for water-quality assessment. Information
<br />about the environmental setting provides a framework
<br />of the basin characteristics and includes natural and
<br />human factors that affect the physical, chemical, and
<br />biological quality of the water in the basin. This infor-
<br />mationcan be used to design data-collection studies in
<br />the study unit for the NAWQA program and can aid in
<br />determining the effects of natural conditions and
<br />human factors on water quality in the basin.
<br />The study unit has a drainage area of about
<br />17,800 mil, and the primary river within the basin, the
<br />Colorado River, originates in the mountains of central
<br />Colorado and flows about 230 mi southwest into Utah.
<br />The major tributaries to the Colorado River in the study
<br />unit are the Blue, Eagle, Roaring Fork, and Gunnison
<br />Rivers. In 1990, population in the basin was about
<br />234,000. The largest population center in the basin is
<br />in the area around Grand Junction.
<br />Climate in the basin varies from alpine condi-
<br />tions in the eastern part to semiarid in the western part.
<br />Mean annual precipitation ranges from more than
<br />40 in. at the higher altitudes to less than 10 in. in the
<br />lower altitudes of the basin. The geology predomi-
<br />nantlyconsists ofcrystalline rocks of Precambrian age,
<br />stratified sedimentary rocks, and alluvial deposits.
<br />Land designated for use as rangeland or forest
<br />accounts for about 85 percent of the use in the basin.
<br />The other major land uses in the basin are agriculture,
<br />mining, and urban. Water used from surface water
<br />for irrigation accounts for about 97 percent of the total
<br />offstream water use. Ground water, which accounts for
<br />less than 1 percent of water used, typically is used for
<br />domestic purposes in the rural parts of the basin.
<br />Study unit streams are classified in this report
<br />into three general types: high-altitude streams, low-
<br />altitude streams, and mixed-type streams. Annual,
<br />seasonal, flood, and low flows in these three stream
<br />types vary considerably in the basin. The natural
<br />hydrology has been extensively altered by water devel-
<br />opment,which includes numerous reservoirs and
<br />diversions.
<br />In the study unit, ground-water resources have
<br />not been extensively developed. The most productive
<br />wells come from alluvial aquifers in the basin. Other
<br />ground-water resources include consolidated aquifer
<br />systems in sedimentary rocks and fractured systems
<br />such as those in metamorphic and granitic rocks.
<br />Algae, fish, and macroinvertebrates that are
<br />characteristic of the Southern Rocky Mountains and
<br />Colorado Plateau physiographic provinces vary with
<br />altitude and physical habitat. The dominant algae in
<br />the high-altitude streams are blue-green and green
<br />algae. In the lower altitudes, golden-brown and green
<br />algae are predominant. Cold water species, such as
<br />trout, are present at the higher altitudes, and warm-
<br />water species, such as carp, catfish, minnows, and
<br />suckers, dominate the lower altitudes. In the higher
<br />altitudes, caddisflies and mayflies are the dominant
<br />macroinvertebrates. The lower altitudes are favorable
<br />to species such as leeches and snails.
<br />Natural and human factors affect the physical,
<br />chemical, and biological characteristics in the basin,
<br />which then affect the water quality. The natural weath-
<br />ering processes of a variety of geologic formations in
<br />the basin add salts, minerals, radionuclides, and trace
<br />elements to the surface and ground waters. Interbasin
<br />water transfers along the Continental Divide decrease
<br />the quantity of water in the headwater streams and the
<br />dilution capability of these streams. Water quality in
<br />the headwater streams along the Colorado Mineral Belt
<br />is being degraded by past mining activities that affect
<br />aquatic life. Urbanization and recreational activities
<br />are increasing throughout the basin and have a marked
<br />effect on the quantity of water needed as well as the
<br />quality. In the lower part of the basin, agriculture has a
<br />major effect on the quality of surface and ground
<br />waters predominantly because of return flows from irri-
<br />gation.
<br />REFERENCES CITED
<br />Ackerman, D.J., and Brooks, Tom, 1986, Reconnaissance
<br />of ground-water resources in the North Fork Gunnison
<br />River Basin, southwestern Colorado: U.S. Geological
<br />Survey Water-Resources Investigations Report
<br />85-4230, 21 p.
<br />Apley, Martyn, 1982, Powderhorn surface water quality
<br />study: Gunnison, Colorado Western State College,
<br />46 p.
<br />SUMMARY 31
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