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<br />communities can be compared, The major controlling
<br />factors that determine composition of biological
<br />communities are changes in water tcmperature, water
<br />discharge and velocity, substrate and suspended
<br />matcrial, 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 />rcsult ofa combination of these factors,
<br />
<br />IMPLICATIONS OF ENVIRONMENTAL
<br />SETIING ON WATER QUALITY
<br />
<br />Water quality in the Upper Colorado RiverBasin
<br />is affected by the physical, chcmical, and biological
<br />characteristics that make up 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 />V,S, Geological Survey (I 988); Liebermann and others
<br />(1989); and Colorado Department of Health (1992),
<br />
<br />Natural Factors
<br />
<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 fornlations 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 Precanlbrian 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 />ern part 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 50\;0<,
<br />Water quality also can be affected by the chemis-
<br />try of precipitation, In the study unit, there are consid-
<br />
<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 G lenwood
<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 in high-mountain
<br />streams (J,T, Turk, U,S, Geological Survey, oral eom-
<br />mun" 1995), In the Colorado Plateau, the aonospheric
<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 />
<br />Geologic formations affect waterquality because
<br />rocks are the source of many chemical constituents in
<br />the water, Soluble salts, minerals, and trace element~
<br />from different geologic formations result in increased
<br />dissulved-solids concentration and determine the
<br />chemical composition of the streams, In the Suuthern
<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 uther salts that are present in
<br />this formation affect the water quality, Within the
<br />Piceance Structural Ba~in, the Mancus 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 ofthe study
<br />unit, weathering of authigenic pyrite in the Mancos
<br />Shale results in increased selenium concentration inlhe
<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 />Huwever, 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 \) (Geldon, 1989),
<br />Amounts ranging from 475,000 to 534,000 tons of dis,
<br />solved solids are added annually from the springs to the
<br />surface water (Warner and others, 1985; Liebermann
<br />and others. 1989).
<br />
<br />Soils affect water quality as a source of sus-
<br />pended sediment and soluble materials, Soils in the
<br />
<br />28 Enolronmenlal 5alUng and tmpltcallons on Walar Quality, Upper Colorado Rloar B..ln, Colorado and U\a~
<br />
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