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<br />~cknowleClgments
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<br />The authors thank members of the Upper
<br />Colorado River Basin Liaison Committee for their
<br />cooperation in providing information and data about
<br />the basin, We would especially like to thank Leslie
<br />Simpson from the Colorado Deparmlent of Health and
<br />Environment for providing the National Pollution
<br />Discharge Elimination System (NPDES) data base;
<br />Dav id Dzurochin, Greg Ibarra, and Stephanie
<br />Shoebaeh from the Colorado Deparmlent of Natural
<br />Resources State Engineer's Office for water
<br />diversion infornlation; Robert Crifasi from the
<br />Denver Water Board for reviewing the manuscript; and
<br />Jim Hokit from the Uncompahgre Valley Water Users
<br />Association for water diversion information,
<br />
<br />ENVIRONMENTAL SETTING
<br />
<br />The environmental setting of the Upper
<br />Colorado River Ba~in study unit is described on the
<br />ba~is ofnatural and human factors, Natural factors that
<br />affect background water-quality conditions are physi-
<br />ography, climate, geology, and soils, HWllan factors
<br />that affect background water-quality conditions are
<br />population, land ll~e, water management, and water use
<br />in the basin, Hydrologic and aquatic biological charac-
<br />teristics are affected by natural and human factors,
<br />
<br />Physiography
<br />
<br />The study unit is divided almost equally into two
<br />physiographic provinces: the Southern Rocky Moun-
<br />tains in the eastern part and the Colorado Plateau in the
<br />western part (Hunt, 1974) (fig, I), The Coloradu
<br />Plateau has been divided further intu the Uinta Basin
<br />and the Canyunlands subprovinees on the ba~is uf
<br />geulugic features and formations that arc in thcse areas,
<br />In the eastern part of the study unit, north-northwest-
<br />trending mountains range in altitude from 11,000 to
<br />more than 14,000 ft; these moontains are flanked on the
<br />west by steeply dipping sedimentary rucks, The Con-
<br />tinental Divide marks the eastern and southern bound-
<br />ary orthe study unit. In general, the topography in the
<br />western part of the study unit generally consists of high
<br />plateaus with altitudes ranging frum about 5,500 to
<br />S,500 ft, bordered by steep cliffs along the valleys, The
<br />altitude of the valleys near the Colorado-Utah border is
<br />about 4,300 ft,
<br />
<br />Climate
<br />
<br />Because of large differences in altitude, the
<br />climate in the study unit varies substantially from
<br />alpinc conditions in the east to semiarid in the west.
<br />Daily, monthly, and annual average temperatures arc
<br />highest at the lower altitudes to the west. Mean annual
<br />temperatures range from as low as 32,80F in Gunnison
<br />County near the Continental Divide to as high as
<br />54,IoF near Grand Junction (Benci and McKee, 1977),
<br />Temperatures in the study unit are coldest during Janu-
<br />ary and are warmest during July and August.
<br />Precipitation in the basin ranges from more than
<br />40 in/yr in the eastern mountainous regions to less than
<br />10 in/yr in the western regions, Mountain areas receive
<br />most of their precipitation during the winter when
<br />average seasonal accumulations of snow can exceed
<br />100 in, In the Grand Junction area, the largest amount
<br />of precipitation occurs during August as a resolt of
<br />weather patterns that produce late-afternoon thunder-
<br />sto/llli; (Chaney and others, 19S7), The area~ having
<br />the most precipitation are at the higher altitudes gener-
<br />ally in the ea~tern and southern parts of the study unit
<br />(fig, 2); precipitation decreases toward the lower
<br />altitudes in the western part of the study unit.
<br />
<br />General Geology
<br />
<br />Bedrock geology for the study unit is described
<br />in table I and shown in figure 3; the geology varies
<br />substantially and consists of crystalline rocks of
<br />Precambrian age, stratified sedimenlary rocks, and
<br />alluvial deposits, The following paragraphs give a
<br />brief description of geologic units from oldest to
<br />yoongestthat are most prevalent or have a substantial
<br />implication on water qoality,
<br />Within the central parts of the uplifted mountain-
<br />ous areas in the Upper Colorado River Basin, Precam-
<br />brian metamorphic schists and gneisses that have been
<br />intruded by granitic igneoos rocks are exposcd, The
<br />Precambrian rocks are overlain by sedimentary rocks
<br />of Paleozoic, Mesozoic, and Cenozoic age, Rocks of
<br />Cambrian to Mississippian age primarily are composed
<br />of carbonates (dolomite and limestone) with interbed,
<br />ded sandstones, Rocks oflate Paleozoic age consist of
<br />interbedded shales, sandstones, limestones, and local
<br />deposits of salt and gypsum, Rocks of early to middle
<br />Mesozoic age consist of alternating sandstones and
<br />shale units, Some of the rocks of Jurassic and Creta-
<br />ceous age that are of importance in the study unit are
<br />divided a~ follows: Entrada Sandstone; Morrison
<br />Formation, consisting of varicolored siltstone and
<br />mudstone with beds of sandstone and limestone;
<br />
<br />ENVIRONMENTAL SEmNG 3
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