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<br />Fig. J. NatlfJnf" P~"k,. an" NII"d""' Monumenr.t 'n ,II.. ("OlfJrtlf'fJ IUI'" Bmdn.
<br />
<br />LOWER C01.0RADO RIVER BASIN
<br />
<br />PI, v~/(I"r/I"/IY
<br />The Lower Colorado Region i5 com!,osed or a com!,lcK or !,Iatenll~. mOllnlnin~.
<br />canyons. deserls. and plains. with elevations ranging rrom 75 reet ahove sea level year
<br />Yuma. Arizona, to over 12,600 reel ahove sea level at Humphreys Peak near Fla~staft',
<br />Arizona. The topography takes in virtllally every rorm and degree rrom level plams to
<br />.precipitous mountains and canyons between these elevation extremes. .
<br />Similarly, the geology of the Region includes 8 ~road spectrum ~r sedimentary, meta-
<br />morphic and igneous rocks which !,rodu~ a wide var..e~y or sOils Ioc:ally an~ along
<br />stream counes. In short, the princi!,al phYSical characterIStiCs or the Region are ItS great
<br />variety or land rorms, to!'ollrn!'hy and geolollY., .
<br />Many or the slreall1~.trihulU"y tothc ('olorndo River head 111 narrow ~anyons an~ run
<br />through deeply-cut gorges, The Cohlrudo Riv~r hccal11e.athrough-~loWll1g strell~ m the
<br />late Cenozoic time. Downcutting by the fiver and ItS tributaries resulted In .dee~
<br />entrenchment or the entire system creating spectacular canyons, most notable of wh.Jeh "5
<br />the Grand Canyon or the Colorado, All runolT rrom the high ~Iatealls and mountal~s ~n
<br />the Lower Colorado Rcgion is tributary to the Colorado, Little Colorado .and ~Irgm
<br />Rivers. The Colorado and Virgin Rivers are perennial; all other streams are mtermlttent
<br />at their connuence with the Colorado River except for ~hort reaches downstrea.m from
<br />springs such as Blue Spring and those in Havasu Canyon and near St J~hns, Arizona.
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<br />The Phy.~/cal Enl'/ronmE'nt of thE' Colorado Rh'"r Ba.dn
<br />
<br />19
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<br />The storage and movement of ground water in sedimentary rocks are controlled by
<br />regional structure or the basins and upwarps. Storage and movement in alluvial hasins
<br />are rrequently controlled by valley plugs created by lava nOW5, tectonic movement and
<br />nearly impervious alluvial deposits. The main areas or rechargc are on the highlands, on
<br />the upwarps, and along the structural divides. Movement of ground water is downdip
<br />from the highlands toward the pcrennial streams (Lower Colorado Region State-Federal
<br />Interagency Group, 1971).
<br />
<br />ClimatE'
<br />
<br />The Lower Colorado Region's climate varies widely because or a large difference in
<br />elevation, a considerable range in latitude, and the distribution or mountain range5 and
<br />highlands. Generally, the climate or the more densely !'o!'ulated areas or the Region. such'
<br />as Las Vegas. Nevada, and Phoenix, Tuscon and Yuma. Arizona. is characterized hy
<br />mild tei warm annual temperaturcs, low to moderate humidity and low annual preci!,i-
<br />tation. Summer tempcratures often reach IOO"F and abovc. Rainfall i~ predominantly in
<br />the form or thunder showers which are sometimes very inlense and !,roduce nash nood.
<br />inllloc;llIy. EKce!'t in the mountains. snow is rare and short-lived. Annual rrost-free days
<br />in the lower dcsert range rrom over 270 days to year-long. Yuma. Ari/ona has eXflCri-
<br />eneed several yenrs in succession without n killing fmst. Thesc climatic ractors are very
<br />favllfnhlc 10 Ihe irrigalcd a!:rieuhurc conecnlraled in the lowcr clevalions.
<br />In conlr..st, such cilies as Sl. Gcorge. Ulah. Winslow and lIolhrook. Arizona and
<br />Gallup, New MeKico, have more or a rour-season climate with shorter rrost-rree periods
<br />and colder winter months. FlagstalT. ArilOna. and other cities in mountains, rimrock, and
<br />pl~teall locations have suhstantially shorter frost-free periods. mild summers, cold
<br />wmlers ;lnd considerahly more rain and snow. Most of the recharge to ground water and
<br />the surface runoff in the LCRB results from precipitation in the high mountainous areas.
<br />VI' to 30 inches or !,reci!,illltion a year lire recorded, much or ,,'hich i~ in the rorm or
<br />~noW (Lowcr ('olorado RCllion Stnlc Fedcrulln1ernllcncy GWIII'. 1971)
<br />
<br />Watl'r resnllrce.~
<br />
<br />~he Lower Colorado River Basin ptohably comes closer to using the lasl drop or
<br />available water for man's needs than any other area in the U.S.A. These water supplies
<br />are also a~ong the most regulat~d and metered in the Nation. Despite the most rrugal
<br />use. recycling and the hIghly effiCient rcgulation or water, the lCR B is mining water each
<br />year that cannol. be, re!,Ie,nished by natural recharge ror many decades. Virtually every
<br />sector or the Region s soclo-economic structure is heing sustained in some dq~rcc hy the
<br />overdraft of ground waleI'.
<br />Three main sources of water were availahle in 1965 flowcr Colorado Region Slate-
<br />Federal Interagency Group. 1971): (a) enlitlcmenlto 11 mllr or Colorado River waleI' hy
<br />a body of .Iaw rderre~ ~o as Ihe, "law or the River"; (h) local runolT amounting to
<br />3.12 mar Orlglll;'.'lIlg wlthlll the regIOnal boundaries and (cl lOCAl ground water reserve~.
<br />The 3.1 mar enllllement rrom the Colorado Rivcr has not been rully utili/cd because of
<br />Ihe lack or conveyance racilities to tran~!,ort water 10 Ihe areAS or heAVY demand. At-out
<br />8,2 mar were wilhdrawn from surrace- and ground-water sources in 1965. Apl'roKimately
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