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<br /> <br />H_ .~ e::'" <br />-~", --';19:1- <br /> <br /> <br />--.- ~ <br /> <br />.. - <br /> <br />,. <br /> <br />J <br /> <br />~ <br /> <br />Figure 21. lower Colorado River, Arlzona-Callforma. al <br />Laguna Dam, Upper; NOle continuous riparian vegetation <br />and river being navigated by paddlewheeled vessel In 1906. <br />Lower: Riparian vegetation has been replaced In 1975 by <br />agnculture. and the river flows In a Constrained, dredged <br />channel. Photographs prOvided by US Bureau 01 Reclama. <br />tlon and A D. Ohmarl. respectively <br /> <br />material formult: rht" bonom at d .c1\'en pl.l('" <br />consunrly exchanged. I{ D,l]anu.' shifts to [he POSltl\'t' <br />side. the streJrn channel wdl huild up. or J.gf:r.ldl:. A <br />neg.1tI\'e shlit results In 101;.11 erosIOn. or dl'.craJJuon. <br />In the 1 HO()s. mdny southwe5(,'rn strt".lffiS had <br />a~radeJ. and thw;ed oeM the tOps l)! rht'"lr D.lnks. <br />ExrenSl\"t.' mHsht's .tnJ flpan;ln plant (l)mmUnLtLes <br />slowed water (urrents. prllmotJn~ Jl:posltlon 01 <br />lH~,lnL( and lflorF.inLC m.itt'rlals (FI!'_ ~~). BCdH'r <br />were common. ..nJ tht'lr d.lms conrriburt:J to o\"('r.all <br />stabilization of the systems layers o! ort:anl( Jeprls <br />lflterheJded With ;.!Iunum ,lnd laced rogetht'r ~~. the <br />trunks ,lnd Itmb" ot dl)Wn rret'~ sO.lked up tlnoJ <br />water, which SIIlW!y le.l!.;ed lOW the channd dUClnF <br />dry times. thereby bul(eTln~ r1w Imp.l(H o! dwu~ht. <br /> <br /> <br />FIgure 22 Marshlands assoclaled With Babocomari Creek, <br />upper San Pedro River baSin, Arizona, 1968. This is one of <br />the few, relatively undisturbed cmnega habitats remaining in <br />the American Southwest. <br /> <br />A maJor car,lsrrophe o((urred In rhe 1870s. <br />ll\.t's[0ck. mtrodun:d ro the re~lOn by ranchers, had <br />becoml: ,lhunJanr. A:. rht: uplands dt:siccated. canle <br />(oncentr.!teJ nt:<lr streams and rlvers. Bur even rhat <br />t;InK suon 1;II It'd. and 75 q- of all livestock 10 <br />Arizona are thou~hr to ha\'e Jlt"d of thirst or <br />sr.lC\.atlon by ll'PS. R.lnFt's were st"\'erely damaged. <br />so t"roslOn prt:\J.ilt"J when ;1 wer cycle be~an in the <br />11;805. Dt"t'p afw}"os wert' cut from downstream to <br />upsrream. inClsin~ \'alley tills so deeply that wart:r <br />tables wert' drained (Fig. 2.'; St'e also Figs. 7, 19). <br /> <br /> <br />Figure 23. Clenega Creek, Arizona. 1977: organiC depoSits <br />(dark bands) formed dunng periOdS of marshland <br />development, are Interbedded With alluvium depoSIted from <br />the watershed (light bands) In eroded banks of many inCIsed <br />Southwestern streams <br /> <br />12 <br />