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1. INTRODUCTION The Colorado squawfish (Ptychocheilus lucius) spawns at a limited number of sites on the falling limb of the annual snowmelt runoff hydrograph (Figure 1.1). Field investigations in 1991 and subsequent geomorphic and hydraulic analyses of the Colorado squawtish spawning bar at C1eopatras Couch at River Mile (RM) 16.5 (Figure 1.2) in the lower Yampa Canyon led to the development of a physical process-biological response model (PRM) for spawning habitat formation (Harvey et aI., 1991, 1993; Mussetter et al., 1992). Sediment deposition and bar formation occur at discharges greater than 10,000 cfs, a discharge at which downstream hydraulic controls cause backwater and reduced sediment transport capacity of the flows. Spawning habitat at this location is formed by bar dissection and erosion at a range of flows between 400 and 5,000 cfs when the local hydraulic energy is greatest because of reduced tailwater downstream. Sediment delivery to the eroded chute channels is reduced by deposition in an upstream pool, the result of velocity reversal between pools and riffles during recessional flows (Keller, 1971; Usle, 1979). Fish capture data indicate that ripe and tuberculated fish are only present at specific locations around the spawning bar when specific discharge related physical and hydrodynamic conditions occur at those locations. A significant finding of the 1991 investigation was that flushing of fines to form the clean cobble substrate occurs during recessional flows and not during the peak flows as had been assumed previously (O'Brien, 1984; Tyus and Karp, 1989; Tyus, 1990, 1992). However, bar building is dependent on the occurrence of high discharges, and the question of the required frequency of the high discharges could not be addressed quantitatively with the avaUable data. On a conceptual level it is evident that constant dissection of the bar without redeposition induced by higher magnitude discharges would eventually lead to conditions where spawning habitat would not be available. The branch and chute channels would widen until the combination of coarsening of the surface bed material (armoring) and reduced unit discharges in the individual chute channels would prevent further reworking of the primary bar sediments, and hence formation of the tertiary bars that appear to be used by the fish for spawning. The process-response model has three primary components. First, 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 increased hydraulic gradient during recessional flows with the downstream hydraulic control being spatially fIXed by erosion resistant material. The high discharges must be able to entrain and transport the bar forming gravels and cobbles to the backwater- affected location. Second, on a mesoscale level, the backwatered and confined flows must cause a bar to be formed that provides for both the resting-staging and deposition-fertilization phases of spawning during the recessional flows, when the local hydraulic energy increases. Finally, on a microscale level, 1.1 Resource Consultants & Engineers, Inc.