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probably represent the maximum bar elevation. Five cross sections originally sUlVeyed for IFIM modeling purposes in 1983 were resulVeyed in 1991 and again in 1993 at the C1eopatras Couch spawning bar. Cross section 1 is located at the downstream end of the bar and XS 5 is located at the upstream end of the bar (Figure 3.15). The superimposed cross section plots are presented in Appendix B. In general terms the cross section data indicate that the chute channels widened and deepened to some extent between 1983 and 1991. Peak flows in 1983 were high (- 23,000 cfs) and bar building took place. It is reasonable to assume that bar building also occurred during the flood of record in 1984 (32,300 cfs) but no sUlVeys were made after the runoff season. Between 1991 and 1993 most of the cross sections plots indicate that there was minor filling of the thalweg of the right chute channel, but no major topographic changes occurred as a result of the 1993 runoff season (Qpk = 17,600 cfs). Six Wolman Counts were made at various locations around the spawning bar during the 1993 field work (Figure 3.16). At five of the locations Wolman Counts had been performed in 1991. The gradation cUlVes for both the 1991 and 1993 grain size measurements are presented in Appendix A. Table 3.2 summarizes the grain size distribution parameters for the 1991 and 1993 data. The data indicate that at all of the sites the bar sediments were finer in 1993 than in 1991. This suggests that in fact there was a rejuvenation of the bar in 1993 even though the sUlVey data indicate very minor topographic changes. The presence of the finer gravels indicates that they were mobilized from upstream sites and deposited as a result of local backwater conditions at the spawning bar. Reduced grain sizes at the bar will enable lower flows in subsequent runoff seasons to continue creating spawning habitat. The relatively small magnitude of the 1993 peak discharge in relation to those in 1983 and 1984 suggests that provided that stored sediments are mobilized in the reach upstream of the spawning bar there is no need for the very large magnitude discharges for the purposes of providing squawfish spawning habitat. The relationships between backwater generation during a range of high flows at the bar sites and supply and mobilization of sediment in the reaches upstream of the backwater influence require further investigation if the process of formation and maintenance of spawning habitat is to be fully understood. 3.3. Water Chemistry The possible effects of water chemistry on chemoreceptive imprinting of Colorado squawfish (Wick et at, 1983; Tyus and Karp, 1989; Tyus, 1990) or as spawning cues (Nesler et aI., 1988) have been suggested for some period of time. Further, the presence of the flocculated mud and the natural mortar at Mathers Hole suggest that there are water chemistry characteristics that bear some investigation. Umited water chemistry data were collected on the Yampa River by the USFWS in 1981 3.28 Resource Consultants & Engineers, Inc.