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<br />The UCR Band LCR B are each subdivided into three hydrologic divisions as shown in
<br />Fig. I, The UCRB consists of the Grand, Green, and San Juan divisions, while the LCRB
<br />consists of the Lower Main Stem, Little Colorado and Gila divisions.
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<br />. UPPER COLORADO RIVER BASIN
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<br />The boumlury of Ihe- Up/lCr Colomdo River D,u;n from "lL'C Ferry", Ari1Ona, north-
<br />ward follows Ihe crests of the Pariu, Aquarius, Blld Wasuleh Plateaus Bnd the Wasatch
<br />Bnd Wyoming Ranges to the Continental Divide at the north end of the Wind River in
<br />Wyoming. The UCRB boundary from "Lee Ferry" southward and eastward follows a
<br />divide that trends lirst southward and Ihen eastward across the Kuibi/o Pla/eau, along
<br />the north and easl rim of Black Mesa. and across the south end of the Chuska Moun-
<br />tains to the Continental Divide a few miles northeast of Gallup, New Mexico. From here
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<br />Tile PIr.y.,ical Envlronmf'lit oflhe Colorcufo Ri,'er Basin
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<br />northward, the east boundary of the basin follows the Continental Divide almost 1000
<br />miles /0 tbe north end of the Wind River Range (Iorns 1'1 al.. 19(5).
<br />The Colorado River riscs ncar Ihc C~lrcme easlern part of Ihe UCRB on the east slope
<br />of Mount Riclllhofen, a peak having an allilude of 13,000 fcct on the Continental Divide
<br />and flows generally southwestward 10 "Lee Ferry", The Green River, the largest tribu.
<br />tary, rises in the Wind River Runge at Ihe norlh cnd of the basin and flows
<br />southward to ils junction wilh the C%ra do River aboul 60 miles south of rhe town of
<br />Green River, Utah. The San Juan River, the second largesl tribulary, rises on the west
<br />slope of the Conlinenlan)ivide inlhe southeastern part of the busin and flows westward
<br />to its junction with the Colorado aboUI 75 miles west of Bluff, Utah: , .
<br />The principal tributaries of lhe Colorado River above the Green RIver (m earher years
<br />the river above the mouth of the Green River was called Grand River, but in 1921 Grand,
<br />was changed to Colorado) are the Eagle River, Roaring Fork, Gunnison River and lhe
<br />Dolores River. The principal tributaries of the Green River are New Fork River, Big
<br />Sandy Creek, macks Fork. Henrys Fork. and Yampa, While, Duchcme, Price and San
<br />Rafael Rivers. The principaltrihutaries of the San Juan River arc the Navajo, Los Pinos,
<br />Animas and La Plata Rivers. Glher tributaries thaI enter the Colorado River below the
<br />Green River are Ihe Dirly Devil. Escalanle. and Paria Rivers (Iorns ('/ 01.. 1965).
<br />The plateaus and mountains that form Ihe "oundaries of the Upper Colorado River
<br />Basin and the highlllnds in Ihe inlerior are a series of uplifted earth masses deeply
<br />dissected by erosion, by glaciation and by wealhering. Between Ihe intersecting mountain
<br />ranges in the interior of the "asin are plaleaus. mesas, and hroad basins. some gently
<br />rolling and olhers deeply carved by erosion,
<br />Long before the earlh mm'emenls lhal created l,he presenl mountains started. the area
<br />WllS the <wne "r allernale enl.toadlmenl and relreat ,'f greal inland seas. The <,'dimentary
<br />material Ihat aCl:lInllllall'd and Was not slIbscquenlly removed by erosion during the
<br />periods when Ihe land stood above the seas is represented by Ihe sedimentary rocks that
<br />lie bcnealh much or the basin. These rocks arc thousands of feet thick and range in
<br />altitude from sharply tilled aHlund Ihe mountains 10 nearly horizontal in Ihe interior.
<br />These evenls look place during Ihe Pale%ic and Mes%ic Eras.
<br />Earlh movement Ihat formed the present mounlains began in Ihe Mesozoic and con.
<br />linued inlO (he Cenozoic Era, These movements formed the ancestral Rocky Mountains
<br />and slarted regional downwarps. Which culminated in at least si~ large struclural basins.
<br />These basins received thick deposits of sediment eroded from the highlands. During
<br />middle Cenoloic time slreams began to downeut into the Cenoloie and older Pale07:oic
<br />and Mesoloic rocks. Conlinuous erosion since middle Cenozoic time has !"roduced the
<br />present topogra!"hy (Iorns rr "'., IQ6SI.
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<br />The c1imale of the Upper Colorado River Basin is due more to the influence of
<br />mountain ranges in the movement of air masses than to latitude. The high mountains are
<br />comparatively wet and cool. whereas the plaleaus and lower mountains are dryer and are
<br />subject to wide ranges of temperature; The interior valleys al lower a/Wudes are hot and
<br />dry in the summer and cold in the winter,
<br />Moist Paeilic air masses can move across the entire basin. Dry polar air from the
<br />north and moist tropieal air from the south move into the basin at times, but rarely
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