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communities can be compared. The major controlling
<br />factors that determine composition of biological
<br />communities are changes in water temperature, water
<br />discharge and velocity, substrate and suspended
<br />material, chemical conditions, and aquatic and riparian
<br />vegetation (Ward and Kondratieff, 1992). The differ-
<br />ences in biological communities between the Southern
<br />Rocky Mountains and the Colorado Plateau are the
<br />result of a combination of these factors.
<br />IMPLICATIONS OF ENVIRONMENTAL
<br />SETTING ON WATER QUALITY
<br />Water quality in the Upper Colorado River Basin
<br />is affected by the physical, chemical, and biological
<br />characteristics that makeup the environmental setting.
<br />The availability of mineral and organic materials to the
<br />hydrologic system affects the water quality in the
<br />basin. Materials dissolved in or removed from the
<br />water can result from natural or human factors. In this
<br />section, the major natural and human factors that affect
<br />regional water quality are described in the context of
<br />the environmental setting. A general description of the
<br />occurrence and distribution of selected constituents is
<br />based on summaries by Chaney and others (1987);
<br />U.S. Geological Survey (1988); Liebermann and others
<br />(1989); and Colorado Department of Health (1992).
<br />Natural Factors
<br />Climate is an important natural factor affecting
<br />water quality as a result of changes in altitude, precipi-
<br />tation, runoff, and evaporation. The weathering pro-
<br />cesses of geologic formations are affected by air
<br />temperature, which is a function of altitude and distri-
<br />bution of precipitation in the basin. In the higher
<br />altitudes, precipitation can exceed 40 in/yr; however,
<br />the Precambrian rocks and Tertiary volcanic rocks
<br />exposed at these higher altitudes are fairly resistant to
<br />weathering, and the concentrations of dissolved solids
<br />in the water are limited in these upper stream reaches.
<br />When the streams come in contact with outcrops of
<br />sedimentary rocks in the middle and lower reaches, the
<br />dissolved-solids concentrations in the water increase.
<br />In the more arid climate at lower altitudes in the west-
<br />ernpart of the basin, precipitation commonly in the
<br />form of thunderstorms generally is less than 10 in/yr,
<br />but the thunderstorms can mobilize runoff of large
<br />loads of sediments and solutes to the streams. In addi-
<br />tion, evaporation in the semiarid to arid climate
<br />enhances the precipitation of dissolved solids.
<br />Water quality also can be affected by the chemis-
<br />try of precipitation. In the study unit, there are consid-
<br />erable chemical inputs from atmospheric deposition of
<br />nutrients and major ions. Two National Atmospheric
<br />Deposition, Program sites for monitoring constituents
<br />in the atmosphere are located near the Glenwood
<br />Springs area, and a number of other sites surround the
<br />basin. The loading of nitrate in the mountainous areas
<br />near the basin can be an important part of the nutrient
<br />budget for high-altitude lakes. Also, concentrations of
<br />nitrate in precipitation are several orders of magnitude
<br />greater than concentrations measured inhigh-mountain
<br />streams (J.T. Turk, U.S. Geological Survey, oral com-
<br />mun., 1995). In the Colorado Plateau, the atmospheric
<br />contribution is secondary to the human effect from
<br />agricultural and municipal sources. Atmospheric
<br />inputs could be a source of metals and organic com-
<br />pounds, especially in the mountainous areas of the
<br />basin, but their significance has not been documented.
<br />Geologic formations affectwaterqualitybecauw
<br />rocks are the source of many chemical constituents in
<br />the water. Soluble salts, minerals, and trace elements
<br />from different geologic formations result in increased
<br />dissolved-solids concentration and determine the
<br />chemical composition of the streams. In the Southern
<br />Rocky Mountains province, water-quality conditions
<br />are related to trace elements, which occur naturally in
<br />geologic formations and soils-most commonly cad-
<br />mium,copper, lead, and zinc. In the Eagle River Valley
<br />where the Eagle Valley Evaporite is exposed, moder-
<br />ately soluble gypsum and other salts that are present in
<br />this formation affect the water quality. Within the
<br />Piceance Structural Basin, the Mancos Shale and
<br />Mesaverde Group are present, and weathering of these
<br />units adds dissolved-solids concentration to the surface
<br />and ground water. Also, in the western part of the study
<br />unit, weathering of authigenic pyrite in the Mancos
<br />Shale results in increased selenium concentration in the
<br />surface and ground water (Wright and Butler, 1993).
<br />Geologic formations along the Colorado River and
<br />Gunnison Valley have radioactive substances, such as
<br />uranium, radon, and radium, that occur naturally, and
<br />concentrations of these elements are present in the sur-
<br />face and ground water.
<br />However, the single geologic factor having the
<br />most effect on quality of water in western Colorado is
<br />the many mineral springs present in the study unit. At
<br />Glenwood Springs, all springs issue from the Leadville
<br />Limestone or alluvium overlying the Leadville Lime-
<br />stone and Belden Formation (table 1) (Geldon, 1989).
<br />Amounts ranging from 475,000 to 534,000 tons of dis-
<br />solvedsolids are added annually from the springs to the
<br />surface water (Warner and others, 1985; Liebermann
<br />and others, 1989).
<br />Soils affect water quality as a source of sus-
<br />pended sediment and soluble materials. Soils in the
<br />28 Environmental Setting and Implications on Water duality, Upper Colorado River Basin, Colorado and Utah
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