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001032 ", Grat's Intffil retation of the effects of tamarisk colonization on the frequency of overbank flooding does not take into account the substantial modification of cross section geometry that occurs in sand bed streams und r high flow conditions. When the channel banks are relatively erosion resistant (Anthony and Ha ey, 1991). Evidence of cross section adjustments occurring on the Green River with changes in discha~e is provided by the repeat cross section surveys at the Cottonwood Bottom IFIM site. I' If Grafs d978) interpretation of the effects of tamarisk colonization are correct then it appears that under natUral~re-dam and pre-tamarisk conditions there was very little overbank larval stage habitat along the I er Green River. This suggests that Razorback sucker laMle that were hatched just before or near the peak of the annual hydrograph (Tyus and Karp, 1989) and drifted downstream to this reach of the river ould have been flushed on downstream. It should be recllgnized that this reach of the Green River is bedrock-constrained reach and that there was never a wide alluvial floodplain &Ven prior to the I uction of tamarisk. From the point of view of the Colorado Squawflsh larvae the frequency of ave nk .floodlng Is moot beeause the squawfish spawn on the recessional limb of the hydrllgraph (Tyus and Karp, 1989), and therefore, under natural conditions they could not have utilized out-of-bank hab' . The reduced channel Width may have an effect on the squawflsh larvae if thli low flow habitat has b en significantly affected. ' Our obse ions of the distribution of riparian vegetation along the Green River from RM 120 to RM 28 (conflu nce with the Colorado River Is RM 0) are somewhat at variance with those of Graf (1978). There ap rs to be a-range of conditions along the reach. In some,locatlons dense stands of willows of su ntlal age and size comprise the riparian vegetation. At other locations the riparian vegetation Is co prlsed of both tamarisk and willows, and at yet other locations the vegetation Is comprised exclus e1y of tamarisk. Given the amount of time available to observe these variations In species comrosit on, we were unable to determine any associations between specific geomorphic factors al'ld speci s distribution. Given the~extensive presence of willows and to a lesser extent cottonwoods, in a wide range of geomorphic s ttlngs along the Green River, it Is not Immediately obvious why the common explanation' for th persistence of tamarisk, over willows was generated. It has been suggested that tamarisk persists ~Urlng floods because flows are Incapable of eroding the root-reinforced banks. Since willows are also ri arian species whose roots reinforce banks (Gra, y and Ohashi, 1983; Smith, 197,6), and willows have obvl usly reinforced banks al'ld persisted along the Green River, it is not clear why Graf (1978) and others (Hadley, 1961) have suggested that before the advent of tamarisk the native riparian. vegetation was removed by floods and therefore had little lasting geomorphic effect on the channels. Perhaps the dept~ of rooting or root density Is greater for tamarisk than willow. During th course of the field inspection of the Green River, it was observed that tamarisk was being remaved b 2 different mechanisms. At Unknown Bottom a large area of tamarisk had died as a result of pondln of flows In the overbank area, probably In 1986. The depression on the left averbank area was the Infll ed remains of a cutoff channel segment. The cuioff at Bonita Bend (AM 31) had already occurred when Powell traversed this reach of the Green River In 1869. Flows entered the depression throu h a crevasse In the natural levee at about RM 30.6L and were prevented from spilling back to the river by the well developed and tamarisk 'vegetated natural levee. Given the relatively Infrequent occurr nce of overbank flows (see next section for a fuller discussion), it is unlikely that this mechanism for removing tamarisk Is very effective. ~~". Tamarisk was also being removed by lateral erosion of banks upon which it had become established. Sta s of very large tamarisks on the outsides of bends. or where the channel was very deep, were bein undercut by flows even though their roots extended to the water line. Vertical accretion as a re ultof vegetation-induced overbank sedimentation leads to increased bank height. In moderately cohe ive upper bank sediments, bank stability is related to both bank angle and bank height, and therefore wh n bank angle Is increased by fluvial erosion of the less cohesive bank toe sediments, the increased ba k height caused by the vegetation-Induced sedimentation leads to mass failure of the 2 Resource Consultants & Engineers, Inc.