7897

• UCREFRP Catalog Number: 7897
• Author: Apodaca, L. E., N. E. Driver, V. C. Stephens and N. E. Spahr.
• Title: Environmental Setting And Implications On Water Quality, Upper Colorado River Basin, Colorado And Utah.
• USFW Year: 1996.
• USFW - Doc Type: Water-Resources Investigations Report 95-4263,
• Copyright Material: NO

Acknowledgments
The authors thank members of the Upper
Colorado River Basin Liaison Committee for their
cooperation in providing information and data about
the basin. We would especially like to thank Leslie
Simpson from the Colorado Department of Health and
Environment for providing the National Pollution
Discharge Elimination System (NPDES) data base;
David Dzurochin, Greg Ibarra, and Stephanie
Shoebach from the Colorado Department of Natural
Resources State Engineer's Office for water
diversion information; Robert Crifasi from the
Denver Water Board for reviewing the manuscript; and
Jim Hokit from the Uncompahgre Valley Water Users
Association for water diversion information.
ENVIRONMENTAL SETTING
The environmental setting of the Upper
Colorado River Basin study unit is described on the
basis of natural and human factors. Natural factors that
affect background water-quality conditions are physi-
ography, climate, geology, and soils. Human factors
that affect background water-quality conditions are
population, land use, water management, and water use
in the basin. Hydrologic and aquatic biological charac-
teristics are affected by natural and human factors.
Physiography
The study unit is divided almost equally into two
physiographic provinces: the Southern Rocky Moun-
tains in the eastern part and the Colorado Plateau in the
western part (Hunt, 1974) (fig. 1). The Colorado
Plateau has been divided further into the Uinta Basin
and the Canyonlands subprovinces on the basis of
geologic features and formations that are in these areas.
In the eastern part of the study unit, north-northwest-
trending mountains range in altitude from 11,000 to
more than 14,000 ft; these mountains are flanked on the
west by steeply dipping sedimentary rocks. The Con-
tinental Divide marks the eastern and southern bound-
ary of the study unit. In general, the topography in the
western part of the study unit generally consists of high
plateaus with altitudes ranging from about 5,500 to
8,500 ft, bordered by steep cliffs along the valleys. The
altitude of the valleys near the Colorado-Utah border is
about 4,300 ft.
Climate
Because of large differences in altitude, the
climate in the study unit varies substantially from
alpine conditions in the east to semiarid in the west.
Daily, monthly, and annual average temperatures are
highest at the lower altitudes to the west. Mean annual
temperatures range from as low as 32.8°F in Gunnison
County near the Continental Divide to as high as
54.1°F near Grand Junction (Bend and McKee, 1977).
Temperatures in the study unit are coldest during Janu-
ary and are warmest during July and August.
Precipitation in the basin ranges from more than
40 in/yr in the eastern mountainous regions to less than
10 in/yr in the western regions. Mountain areas receive
most of their precipitation during the winter when
average seasonal accumulations of snow can exceed
100 in. In the Grand Junction area, the largest amount
of precipitation occurs during August as a result of
weather patterns that produce late-afternoon thunder-
storms (Chaney and others, 1987). The areas having
the most precipitation are at the higher altitudes gener-
ally in the eastern and southern parts of the study unit
(fig. 2); precipitation decreases toward the lower
altitudes in the western part of the study unit.
General Geology
Bedrock geology for the study unit is described
in table 1 and shown in figure 3; the geology varies
substantially and consists of crystalline rocks of
Precambrian age, stratified sedimentary rocks, and
alluvial deposits. The following paragraphs give a
brief description of geologic units from oldest to
youngest that are most prevalent or have a substantial
implication on water quality.
Within the central parts of the uplifted mountain-
ous areas in the Upper Colorado River Basin, Precam-
brianmetamorphic schists and gneisses that have been
intruded by granitic igneous rocks are exposed. The
Precambrian rocks are overlain by sedimentary rocks
of Paleozoic, Mesozoic, and Cenozoic age. Rocks of
Cambrian to Mississippian age primarily are composed
of carbonates (dolomite and limestone) with interbed-
ded sandstones. Rocks of late Paleozoic age consist of
interbedded shales, sandstones, limestones, and local
deposits of salt and gypsum. Rocks of early to middle
Mesozoic age consist of alternating sandstones and
shale units. Some of the rocks of Jurassic and Creta-
ceous age that are of importance in the study unit are
divided as follows: Entrada Sandstone; Morrison
Formation, consisting of varicolored siltstone and
mudstone with beds of sandstone and limestone;
ENVIRONMENTAL SETTING