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-. sional flows. The flushing flows at RM 16.5 are- in the range of 500-5,000 cfs, depend- ing on the location (Figures 10-12). It is within this range of discharges that ripe or tuberculated squawfish have been cap- tured at this spawning bar (Wick et al. 1983; Tyus and Karp 1989; Tyus 1990). As dis- charge decreases, the tailwater down- stream is lowered and local hydraulic slope increases (Figures 7 and 8). Dissection of the primary bar to form the secondary bar, and then dissection of the secondary bar to form the riffles or tertiary bars in the branch channels result in a progressive re- moval of the fines (Figure 4). The origi- nally deposited polymodal clast-support- ed, very poorly sorted matrix conglomerate is progressively reworked to form the clast- supported open-work conglomerate (clean substrate) that is required for egg adhe- sion. Flushing of the matrix fines to form the clean cobble substrate is also depen- dent on the reduction in sediment delivery to the branch channels from upstream as the discharge progressively decreases. The upstream pool becomes a site of sediment deposition and storage rather than erosion and transport of sand- and gravel-sized sediment because the velocity of the flow decreases substantially below 5,000 cfs. Tuberculated and ripe Colorado squaw- fish were only captured at the RM 16.5 spawning bar in resting-staging areas as- sociated with the tertiary bars where the dimensionless grain shear stress (T*') sig- nificantly exceeded a value of 1 at a dis- charge of 1,200 cfs (Figures 10-12). This suggests that spawning habitat requires some movement of the cobbles to prevent redeposition of fines into the intercobble pore spaces from the high turbidity flows, because deposition of the fines in the in- tercobble pore space is dependent on the concentration of the suspended load and is independent of velocity (Einstein 1968). A critical and yet unanswered question is the required frequency of the high dis- charge bar-forming or -maintaining event. Constant dissection of the bar without re- deposition induced by higher magnitude discharges would eventually lead to con- ditions where spawning habitat would not be available. The branch and chute chan- nels would widen until the combination of coarsening of the surface bed material (armoring) and reduced unit discharges (i.e., lower energy) in the individual chan- nels would prevent further reworking of the primary and secondary bars and, hence, the formation of the tertiary bars appar- ently used for spawning. Comparison of photographs of the RM 16.5 bar taken in 1981 before the very high peak discharges in 1983 ( - 23,000 cfs) and 1984 (-33,000 cfs) and those taken in 1991 following a 7-yr period of primarily wet (>13,500 cfs) to average (-11,000 cfs) con- ditions indicates that the widths of the branch channels and the secondary bar dis- secting chute channel were much greater prior to the high-discharge years. Repeat surveys of the cross sections at RM 16.5 in 1984 and 1991 indicate that there has been significant channel widening and lower- ing of the bed elevations in the branch channels (J. S. O'Brien, FLO Engineering, Inc., written communication). Thus, the photographic and topographic compari- sons indicate that the bar is eroded with time following a very high bar-building discharge. As yet, however, there are in- sufficient data to estimate the required magnitude and frequency of bar-building events. Continued monitoring of the chan- nel geometry and sediment gradations within the branch channels would provide the data necessary to evaluate through hy- draulic modeling when the incipient mo- tion criteria necessary for spawning habi- tat formation could no longer be met by the annual recessional discharges. Fish- capture data would be required to validate any conclusions regarding formation and utilization of spawning habitat. A MODEL FOR IDENTIFYING COLORADO SQUAW FISH SPAWNING HABITAT A physical process-biological response model for determining the physical re- quirements for Colorado squawfish spawning habitat can be formulated at three scales: 1. On a macroscale level, the river must be confined so that high discharges do not disperse over a floodplain. The geometry of the reach must cause backwater during high discharges and I M. D. Harvey et al. 127 II~