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<br />OOil7J2 <br /> <br />((hrnundson 1999a) <br />Clean cobble and gravel Sl.lbSHales of <br />upper reaches also provide5Uitable spawn_ <br />ing sites for Colorado pikeminno", As <br />eggs deposited inlhesearea.shateh, emerg. <br />ing larvae drift downstream wilh lhecur- <br />renl In Ihe Colorado Rh'er, mOSI pikem;n- <br />no"'larvaeendupdownstrcamof~oab, <br />Ulah ",here the gradienl naltrns and the <br />river slo"'s (Trammell and Chart 1999) <br />There, sill and sand is dcposited creating <br />barsandassociatedbackwalt'ts Backwat- <br />crsBrewarm, pf<lductive, S1il1-water habi- <br />tats well suited for rearing ofyoungColo- <br />radopikerninnow Iluringlheirfirstyearof <br />life, Colorado pikcminnow C<Jl'lCetllrate in <br />backwaters and feed on zooplankton and <br />chimnomid larvae (Muth and Snyder <br />1995) Later, as C.olorado pikenurJn.ow <br />hecomepiscivonlUS, an abundance of small <br />preyfishC<JntinuC5toprovideampleforage <br />(Osmundsonelal,I998) <br />luColoradopikeminnowreaehBdult_ <br />hood, larger forage fish are prewmably <br />required to maintain body condilion and <br />growth rales However, well forage fish <br />arc scarce in downstream reaches As <br />young adults, Colorado pikeminnow begin <br />10 disperse up and down the river (Fig J), <br />eilher in !;C,8,feh of spawning areas orbeller <br />fceding gmunds Eventually, lhe more <br />abundam supplies of native fish;n the <br />upper river are discovered and adult <br />pikeminnow remain lhefe and establiSh <br />home ranges (Osmundson el al 19'>8) <br />This dispersal pal1em results in somewhBt <br />segrel>BledlifestBgeswilhadulldensilies <br />clumped near the upstream margins oflheir <br />range Cu"ently, the Grand Valley con- <br />lains the greatest densities of adult Colo- <br />rado pikeminnow in the Colorado River <br /> <br />(Fig 4) Also,lhisdistribulionpallernresults <br />in Ihcaveragcsiz.eof Colorado pikeminflOw <br />10 progressively increase in an up~tream <br />direction, Wilh the grealesl average sil.e <br />occurring in the 15.milereach(Fig 5) Thus, <br />lhe importance of this reach is in providrng <br />habilatforadults <br />Compared 10 Colorado pikemionow, <br />~pcdfic allribule~ of the life cycle of the <br />razorback sucker, in terms of it, use oflhe <br />entire river, is largely unknown However, <br />there are somesimilarilies between the two <br />sp<<ies like Colorado pikeminnow, ru-Or- <br />back adulls are generally concenlfBled in <br />upstreamreg;onsoflheC'JI'eenaflflColorado <br />rivers, they exhibit long-range spawning <br />migrations (Modde and Irving 1998), and <br />larvae drill downstream from spawning sites <br />;n the middle and upper Green River {Chart <br />el al 1999) Other evidence su~ests Ihal <br />flooded oollomlands along Ihe Green and <br />Colorado rivershislorieally served BS nurso:ry <br />areas for larvae Ihstorically, bOllomlands <br />were inundated more frequently and for a <br />grealer duralion each spring Ihanoccurs <br />under current river regulation practices <br />(Osmundson and Kaeding 19'>1; Modde <br />1996; Moddeetal 1996) <br /> <br />1M 15-mill' Rt'acn //I P"'''P<'fllw <br /> <br />C'omemporaf)'riverecologyemphasizes <br />lhepropensity ror brodiversily and bio- <br />producliontobclargelYC<Jntrolledbyhabitat <br />main1enance processes such as cut and fill <br />a1luvialion medialed by river discharg.. <br />Along a river's length,environmcntal hetero- <br />gcneily ma"imi,_cs in alluvial reaches, and <br />/1oodplains appear to fuoct;on as ecntersof <br />hiophysical organizalion(Stanfordetal <br />1996) In addition, core populalions(Icla. <br /> <br />, <br />