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. 12/0.7/,92 14: 50 COLO RIVER DIST ~~~ USFWS RIP DNVR ~004 Gmts ~ citheefieds of1mnmisk. calcnizatfon onthefn;lqt.D1cy cf CYBlbank1lcclcfmg does not ~ InIo ;ccoont 1h~ ~ltioJlo modific:a6on d cross sectic:m geometty th;d: occurs in eand bed strtem8 under h{gh flow ~ Vvhen tba channel banb ar8 rulatively erosion resistant (Anthony ard Harvey. 1991). 'Evi:fenc8 of cross seCtIon 4IdJustments ac:curring on the Green Rlverwfth changes in c:iisctIIIge is prcvidec:fbyth8 repeal: cross section stII'Ve)'$ at the Cottonwood BaUcm IFlM site.. 0".0 0 . II BrIts (1978) ~ of1he effects of hal.arfsk coIonrz3IIon - COR8Ct then it appears u.t \K1dIir' nab.nlpntdam and"Pra-famsrisk 0 ccnditbns there was very IJtlle ~ ....... lta(Ie habitat aIcxag the lower Green Rivery o1'hle' wgg~that RaaDrback sucker larVae 1hat were ~ jurt before or neorthe lJeak of the anrlQaIhydrograph (ryus ~ Karp, 1989) Ind drifted cIawn*eam to thrt; rach db riverwadd lave been ~ on downstream. It $hcuJd be reccgnized that1bis reach of 1he Green River is. bedrock-<:onstaUned reach and that thn MS never a wide atuviallacdpbin IMItl priQr to ti1I,J ~ of tarl~ "0 From the. point of view cI the Ccfarmo Squa.wiI$h larvae the frequency of CMI'b;nk fioOOing is. moot ~ tile ~ spawn on the rQCeSSfonaI Umb of the ~ (fyus and Karp, 1!m), and therGfore. under.rwtura QCndftions they could not have utilized oukJND1k habitat. The reduced channel width may haVe an effect on the squawfish laMe If the low' IlCN'l habbt has been signif~ arrected.. Our abseMdicns of the distribution of riparian vegetatIon along the Green River frem AM 120 tQ flU 28 (canIIuiinca wfIh tlle CoIonicIo f1wer is AM 0) are somewhat at ~ with those of GnIf (1978). ~ ~ tQ be. H"1'1Il1S- d conditions lIong the reac:it. In BOrne. focafklns dense and. cI wIIow8 d sub&tantfCII age and .sIzG I;:(Imprise the rip:lrlan vegetation.. Ju. other 10CElti0ns the riparian v~ Is comprised of both tamaffsk Md ~ and at Y. ather 1acati0t'l9 the veg~ is comprised exdusiwl1 of ~Ic. GN<<l tnG! amoUfIt of time avaJrabIe to observe theSe variations In species composition. we were tInlib[e to determine any ClS5QC'btfoM between specifIc: gecmorphic factors and. ~ distribufion... Glen the extensfve presence of willows and to a Jesser extent cottonwcocIs. in a wide range of geomorphic aeWngs akmg the Green Rlwr, it is net immediately clMou6 .,my ~ cammon . ~ fer the persistence oftamaMk CHef' wIIows ~ genenatecl. It has been qgested !hat tamarisk persJsts ckmg floods because 1Iaws ate Incapable of erodfngtha root-reinfcn:ed bank$. Since wIIOWleI n &Iso rip8rian species whose roats reinforcebank$ (Bray and Ohashl 1S83; Smith, 1978).. and wIIaws have ctNIausIy retnforcecl ~ ~ p~ along the Green River. I: 19 not declf why Gmf (1978) and GIher.J (Hadley. 100!) have suggested that befon) h adYGnt ci tamarisk 1he natIVe ~ vegetatiOn was removed by floods and therofora had little Iasijng geQmOrphlc effect on. the chant1Erls. Perhaps the depth of rcoting or root density is grealer for tarnarfsk th;n dcw. DlIIing the course. of the fiefd rnspection of the Green River. I was obs&rVed that 1amariSk was being removed by 2. different mec::h8nisms. M. UnknoWn Bottan a large area of ~ had died as a restJltcrpondIng cfflows in the ovedJank ~~ prot:mhly in 1988. The depression on the left overbank iUea was the infiIIed ~ of a wtoIf ~nnel segment. The cutcff at Bonita Bend (RM 31) had already occurred when Powell ~ this reach of the Green Riwr in 1869. Rows entered t.he depression through B crevasse in thel'1ltUrcll levee al about RM 30.6L and were prevented from spiIJlng back to 1he river by th(!. well dlMJloped and bmarfsk vegefafed natura[ levee.. Given 1I1e relatively infrequent occurrence of overoonk 80we (see next ~on for a fuller discussion), it is unlikely that this mec:hanism far removing tmnarisk 'm very ~ Tamarisk was also beirG nmoved by Iatm8l eroston of banks upon which It had ~ tsi;I~ SIaxfs afvery large tamarrsks on the ClIIskfes at bends, or ~ the d1annef was WJrY deep, were belng undercut by fiO'Mi even though their rool$ Ii!Xfended to the water line. Vertical "-~ A tl nt8UIt ofvegetatforHnduced overbank sedimentatron Iead8 to ~ bank: hetghl In ~ cohesJve upper b:mk sediments, bank stability is related to both bank angle 3nd bank height,. and therefore When bank angle Is increased b)rfi~ ert;l$lon of the less O'JhesIve bank too sedfm~ the increased bank height czused by the vt-getffiion-tnduced'sedimentation leads to tnaSS failure. of1he 2 Resource Consultants & Engineers. Ine.