|
<br />Water Quality is Influenced
<br />by Geologic Factors
<br />
<br />The underlying bedrock in the
<br />Study Unit is made up of crystal-
<br />line and sedimentary rocks (fig. 5).
<br />Alluvium consisting of stream,
<br />landslide, terrace, and glacial
<br />deposits is present in valleys
<br />throughout the basin. Weathering
<br />of the different geologic units
<br />affects water-quality conditions in
<br />the basin. The sedimentary, igne-
<br />ous, and metamorphic rocks con-
<br />tribute material such as salts and
<br />trace elements to the streams. High
<br />concentrations of some materials,
<br />particularly salts, are derived from
<br />the sedimentary rocks, which are
<br />more common in the western part
<br />of the basin. Also, highly mineral-
<br />ized areas in the upper basin con-
<br />tribute trace metals to surface and
<br />ground water. Selenium occurs nat-
<br />urally in the shale bedrock of the
<br />middle and lower reaches of the
<br />basin and is present in surface and
<br />ground water. In addition, mineral
<br />hot springs located primarily in
<br />carbonate rock units in the north-
<br />
<br />central part of the basin contribute
<br />about 13 percent of the total salt
<br />load at the outlet of the UCaL
<br />(U.S. Department of the Interior,
<br />1995; Butler, 1996).
<br />
<br />Water Quality is Influenced
<br />by Land Use
<br />
<br />The UCaL study was designed
<br />to investigate land-use influences
<br />on water quality (see "Study Unit
<br />Design," page 21, for details).
<br />Urban development, mining, and
<br />agricultural were the three primary
<br />land uses investigated in the
<br />UCaL.
<br />Urban development has the
<br />potential to affect the quality of
<br />surface and ground water by add-
<br />ing nutrients, bacteria, pesticides,
<br />hydrocarbons, trace elements, and
<br />salts from point and nonpoint
<br />sources and by changes to the natu-
<br />rallandscape. Urban land use
<br />accounts for only 1 percent in the
<br />UCaL, which has a population of
<br />about 308,000 people (Bureau of
<br />the Census, 1999). By the year
<br />2020, the population is projected to
<br />increase to more than 500,000
<br />
<br />EXPLANATION ~. .J,\
<br />Geology ("9.
<br />~ Igneous and metamorphic lOt .,J:I ~
<br />I!.!l!iI Volcanic and intrusive 40~......."r\ . . J"
<br />D Undivided sedimentary '"t/''' .",
<br />m Shale 1090 ~' . · "\ ..
<br />
<br />~ ~:~~~~;~: (...1"'-.. ....\.. '\ :~Y
<br />~ 1~!.. ~.:"
<br />"....".." ,~
<br />
<br />. "-,
<br />\ '.
<br />\
<br />Ii'" ",#4\ \
<br />o ~O 40 ""I.~ '""_?
<br />b 2'0 40 6'~ KILOMETERS .I
<br />3ao~.,~~
<br />
<br />
<br />Figure 5. The bedrock geology within the UCOL
<br />changes from predominately igneous, metamorphic,
<br />and volcanic rock types in the eastern and central
<br />areas to predominantly sedimentary rock types in the
<br />western areas (Tweto, 1979; Green, 1992).
<br />
<br />
<br />Topography representative of the Southern Rocky
<br />Mountains. (Photograph by Jeffrey Deacon,
<br />U,S. Geological Survey.)
<br />
<br />(Colorado Department of Local
<br />Affairs, 2000). Resident population
<br />increases do not reflect develop-
<br />ment and services for vacation
<br />properties. There are large seasonal
<br />fluctuations in nonresident popula-
<br />tions within the basin due to recre-
<br />ational activities. During the 1996-
<br />97 ski season, more than 9 million
<br />skiers visited ski areas within the
<br />UCaL (Colorado Ski Country
<br />USA, 2000). Effects on water qual-
<br />ity from urban development are
<br />evident in some communities in the
<br />Southern Rocky Mountains.
<br />
<br />Lode and placer mining, a his-
<br />torically significant land use in the
<br />UCaL, was common throughout
<br />the Southern Rocky Mountains.
<br />Streams and ground water have
<br />been affected by point-source mine
<br />discharge and nonpoint-source run-
<br />I
<br />off from mined areas.
<br />
<br />Areas of intensive agriculture are
<br />located primarily in the Colorado
<br />Plateau. Salinity, sediment, nutri-
<br />ents, pesticides, and selenium and
<br />other trace elements are common
<br />constituents in agricultural runoff.
<br />These constituents can have an
<br />adverse effect on the surface water,
<br />ground water, and aquatic life.
<br />
<br />Introduction to the Upper Colorado River Basin 5
<br />
|