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. ODH)S5 Figure 2 shows the peak flow frequency curves (best fit and 95 percent confidence interval) for the period from 1963 to 1990 at the Green River gage. Depending on the n value used (0.03 or 0.035) at the Cottonwood Bottom reach, the bankfull discharge has a frequency of 25 to 28 percent which are equivalent to return periods of between 4 and 5 years. At Unknown Bottom, depending on the n value used (0.03 or 0.035), the bankfull discharge has a frequency of 14 to 20 percent which are equivalent to return periodS of between 5 and 10 years. In contrast, Figure 3 shows the peak flow frequency curves (best fit line and 95 percent confidence Interval) for the period from 1894 to 1962. Depending on the n value used (0.03 or 0.035) at the Cottonwood Bottom reach, the bankfull discharge has a frequency of 50 to 58 percent which are equivalent to a return period of about 2 years. At Unknown Bottom the bankfull discharges have frequencies from 36 to 50 percent which are equivalent to retum periods of 2 to 3 years. The frequency and return period data obviously do not make any allowance for channel morphologic changes that may have been caused by the establishment of tamarisk (Graf, 1978). If in fact Grafs interpretation. is correct, the frequency of overbank flows would have been lower prior to the advent of tamarisk. 3) Habitat Utilization' Potential Historical fish catch data indicate that )wenlle Colorado Squawflsh are abundant in, the lower Green River (G, Smith, USFWS Memo. to Channel Monitoring Crew, 1992) which tends to suggest that there Is adequate )wenlle habitat In the reach. Since the Colorado squawfish spawn on the recessional limb of the annual hydrograph (Tyus and Karp, 1989), and the larvae are distributed downstream by drift It Is possible that the modification of the flow duration curve (see A~rews, 1986) has had an impact on the larval drift distance., Squawflsh are known to spawn In canyon-bound reaches of the lower Yampa River (Tyus and Karp, 1989) and In Desolation Canyon (E. Wick, pers. communication) on the Green River. If the timing of larval emergence coincides with the longer duration mid-range flows, the larvae could in fact be carried farther downstream than would have occurred prior to flow regulation. With a narrower channel, the flow depth for a range of discharges following larval emergence will be higher and it is therefore conceivable that there will be less slackwater habitat for the larvae. Because of the magnitude of the bankfull capacity and the fact that squawflsh spawn after the peak discharge, out-of-bank habitat will rarely, If ever, be available to larval squawflsh in the lower Green River. Increased post-emergence releases from Ramlng Gorge reservoir will tend to be counterproductive with respect to the larval stage of the squawfish. The Razorback sucker spawns on the rising limb of the annual hydrograph (Tyus and Karp, 1989) and appears to have a fairly limited number of spawning locations on the upper Green River and the lower Yampa River (Tyus and Karp, 1990). Larvae are also dispersed by a drift mechanism, but If flows following larval emergence exceed bankfull there is a potential for them to utilize the out-of-bank habitat. In the reach of the Green River that is the subject of this report (RM 120 to RM 28) the frequency of exceedence of the bankfull discharge is low. If It Is assumed that the bankfull capacity estimates for Cottonwood Bottom and Unknown Bottom are reasonably representative for the bottomlands downstream of RM 97 (confluence with San Rafael River) then it is also reasonable to conclude that in most years any Razorback sucker larvae that enter the reach will be passed through the reach. If flow releases from Raming Gorge reservoir fall to increase the total discharge in the river to the point where the bankfull discharge is exceeded for whatever period is required for successful overbank habitat utilization, the increased flows will merely increase the larval drift rate and distance. 4) Recommendations 1. The channel capacity data at both Cottonwood and Unknown Bottoms and the flow duration and flood frequency data indicate that the lower reaches of the Green River are unlikely to be of great, significance In terms of Razorback Sucker larval-stage habitat. Therefore, there does not appear to be much value In future channel capacity. overbank flow monitoring. 5 Resource Consultants & Engineers, Inc.