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<br /> <br /> <br />c..") <br />lX. <br />c".) <br />(,) <br /> <br />PAH.'" -L 1. DESrRIPrTr"J!;....;;- BASn: <br /> <br />P.. ;:;e010r:; <br /> <br />Rocks of illl 9.f':es '"rom ':hose of +"he Archean 11[.''' f+he aIdes' MOwn <br />feolorical ~)eriori~ to the recent alluvial deposits, ~ncludlnf: igneous. <br />sedimentary~ and metamorphic ':J~s, are foun1 in the Colorado River <br />Basin. The hi,.,.Jl Roc~. Moun:'tins 'Which ion-J.na"'_e ~,he +opoj:"raphy 0:" the <br />UPr'er regions are COt:'lfX>se.i of F;TI'ln' .,~s, schis"'s. ,~'11eisscs. 1'3.'18. :md <br />shr..rply folderi. se~limentw-:.; rocks of limes+one, S01'\i:lstone,'11ld shale. <br />Many periods of 'ieposi'ion. erosion, 'U"..i uphenv'il have nlayed 9. part <br />in the present struCQl:re of +.hese m:mntruns. <br /> <br />In contrast to the folded rocks of the mountains which frinf7e the <br />basin, the plateau country of s~~thwestcrn Wyominr. e~stern Utah, and <br />northern J..rizona is composed ~rincip1tily of horizor.tal s~r3.ta of sedi_ <br />n:ent.'1rJ rocks. Slow bu~. con~tant elevation of t.he lMd area has allowed <br />the Color~do River and its tributaries ~o cut narrow, 1eep canyons into <br />the fll"ol.t-toppe,j mesas. This type o~' erosion reaches its culmination in <br />the Grand Canyon ....here the Color"l;io Riyer has cut throu/?h all of the sed- <br />imentary rocks lown to the oldest Archean lO',rMites. <br /> <br />The Lower B?~in is characterized by broe~. f]~t valleys separated <br />by 10..... l!:OWltain ranges. These valleys are filled by large accwnulations <br />of allu....ial deposits. <br /> <br />Sediment removed by constant erosion of ~he Upper areas was depos- <br />ited in Arizona. California, 'll1d Mexico 8J1d now forms the ~'reat ielta of <br />the Colorado River. <br /> <br />Rese:t"'J"oirs Constructed above Lee Fe'!'r'J (Lake Po-...ell. F'1"l!lling Gorge, <br />Fonte-nelle, Navajo, ~rroW' Point, 2.."Id Blue r.lesa), together ....ith Lake <br />Mead '!ownstrea"!l. have caused some major Changes in streE.ro regimen; <br />(II The stream chann~ls inWldated b:. these rese!'"\-"Oirs are no longer <br />subject to natural streau. erosion: (2) the aCCumulation of sediment <br />and vater vithin the reservoirs slows the growth and fLooding of the <br />Colorado River ieUa: 0) floodinr has di!"..inished in many areas; and <br />(4) sections o~ sediment-laden streams have given W~I to clear water <br />strea:ns and lakes. <br /> <br /> <br />The ~~neral concen~ration in r~~f increases frore the headwater <br />areas dO...11stre!'-J!1 and OCcurs in rela~ion .,0 t.he r:eologic character of the <br />terrain across which the Color~do River and its tributaries flow. The <br />geologic fo~.a~ions that 1areely contribute to ~he ~ineral concentra_ <br />tions in nat~al r~noff are evaporites of Paleozoic ~e. shale of Cre. <br />teceous age. "_'1d salt 1'.1).d f:'tpsum of Terti:n:; aF':e. <br /> <br />c <br /> <br />