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, Spawning by the Colorado squawfish has not been observed in the highly turbid flows of the Yampa River (Tyus 1990), but it is believed, on the basis of radio telem- etry tracking data (Wick et a1. 1983), to take place in two phases: a resting-staging phase in a pool or shoreline eddies, and a de- position-fertilization phase on cobble- gravel bars upstream of the resting-stag- ing areas (Tyus and Karp 1989; Tyus 1990). Similar two-phased spawning behavior of the northern squawfish (Ptychocheilus ore- gonensis) has been observed in low-turbid- ity flows (Beamesderfer and Congleton 1981). Many field and laboratory data in- dicate that both the Colorado and the northern squawfish select clean cobble substrates (required for egg adhesion) and low bottom velocities for spawning (Patten and Rodman 1969; Beamesderfer and Con- gleton 1981; Hamman 1981; Tyus and Karp 1989). Midchannel bars located at RM 16.5 and RM 18.5 on the lower Yampa River provide physical conditions (pools and adjacent gravel-cobble bars) to satisfy the reported spawning requirements. However, the mechanism for generating a clean gravel- cobble substrate is unclear because wash load constitutes about 35% of the Yampa River's annual sediment load (Resource Consultants, Inc., unpublished report). Under such conditions, it is likely that a gravel substrate will contain substantial quantities of fine sediment; the rate of fine- sediment accumulation in the gravel de- pends on the concentration of the sus- pended load carried by the stream but is independent of the flow velocity or the amount of material already present within the pores of the gravel bed (Einstein 1968). Removal of fines from the pores of a gravel-bed river is generally called flush- ing. Depth of flushing of fine sediment from a coarse bed without surface sedi- ment mobilization appears to be limited to a depth equivalent to the median coarse particle diameter (O'Brien 1984; Berry 1985), or less (Beschta and Jackson 1979). Such a depth apparently is insufficient to provide the clean cobble substrate (O'Brien 1984) needed for Colorado squaw fish spawning (Tyus and Karp 1989). Further flushing of fines to provide the required spawning substrate requires bed material mobilization (Reiser et a1. 1989). Therefore, identification of discharges required for flushing of fines involves determination of incipient motion of bed material. At the RM 16.5 spawning bar with a median sur- face particle diameter of 75 mm (3.0 in.), O'Brien (1984) estimated that incipient mo- tion would occur at a discharge of about 21,500 cfs (approximately a 20-yr return period event), and Resource Consultants, Ine. (unpublished report) estimated a range of discharge values between 19,500 and 27,500 cfs (approximately 20- to 100-yr re- turn period events). The magnitude of the incipient-motion flows and their associat- ed return periods suggests that the re- quired flushing of fines by this mechanism is a rare event. Annual observations of Colorado squaw- fish larvae in the Yampa River between 1981 and 1988 (Tyus and Karp 1989) con- firm annual spawning success. However, such success would be unlikely given the infrequency of the discharges that create conditions of incipient motion and hence fine-sediment flushing. Therefore, flush- ing of fines may occur as a result of pre- viously unrecognized processes (Harvey et a1. 1991). The primary objectives of our paper are to: (1) identify the physical processes re- sponsible for creating suitable Colorado squaw fish spawning habitat in the lower Yampa River, and (2) describe a physical process-biological response model that represents the physical requirements for Colorado squawfish spawning habitat that could be used to identify potential spawn- ing habitat at other locations within the upper Colorado River basin. PHYSICAL SETTING The Yampa River in northwestern Col- orado and southern Wyoming is a major tributary to the Green River and has a drainage area of about 8,000 mi2 (Figure 2). The Little Snake River is a major tributary I~ to the Yampa River and delivers about 27% of the water and nearly 70% of the sedi- ment load (Andrews 1978). The Yampa Canyon reach of the river extends from Deerlodge Park just downstream of the Lit- 116 Rivers. Volume 4, Number 2 April 1993