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area has been collected from RM 4 to RM 30. However, the area where spawning has been confirmed by presence of running ripe females is much smaller, extending from Cleopatras's Couch (RM 16) to Harding Hole (RM 19) in Yampa Canyon. In Green ~ River, the suspected spawning area in Gray Canyon extends from RM 142 to RM 187, and spawning has been confirmed from Three Fords rapid (RM 154 to RM 156). All other spawning areas in the UCRB fall in the "suspected° category. In the UCR, larval Colorado pikeminnow have been captured between RM 155 and RM 171 in most years since 1982 (Osmundson and Kaeding 1989, McAda and Kaeding 1991 a, Osmundson and Burnham 1991, R. Anderson unpublished data). Larvae have also been collected above RM 171 (Gunnison River confluence) in 1982 (McAda and Kaeding 1991 a) and 1995 (Anonymous 1996b; R. Anderson unpublished data). Aggregations of adult fish during the presumptive spawning period have been recorded between RM 176 and RM 179 (McAda and Kaeding 1991 a), and at RM 169 when ~ running ripe males (but no running ripe females) were captured from a pool at the base of a newly formed riffle (D. Osmundson, unpublished data). In the Gunnison River, an aggregation of radiotagged fish was observed between RM 30 and RM 35 in 1993 and 1994 (Burdick 1995), and larvae were captured downstream of this reach in 1994, 1995, and 1996 (R. Anderson unpublished data). Movements of radio-tagged fish ~ suggest that spawning may occur near Black Rocks, and in the lower Colorado River in Professor Valley (Archer et al. 1985). In the San Juan River, a suspected spawning area has been detected near the "Mixer" (RM 209-215) on the basis of radiotracked movements of 6 fish and the presence of one ripe male (Ryden and Ahlm 1996). ~ Breeding Colorado pikeminnow are most often concentrated in river reaches containing deep pools and eddies used for staging, and submerged cobble/gravel bars used for egg deposition. Within a spawning reach of several miles, the fish may select one or more localized areas for spawning. Radio-tagged fish move from pools or eddies to presumably spawn on bars and then return to the former habitat (Tyus and McAda 1984, Tyus 1990). Similar behavior has been recorded for spawning northern pikeminnow (Beamesderfer and Congleton 1981). Turbid riverine conditions have precluded direct observation of egg deposition. However, cobbles removed from spawning substrate in the Yampa River during this time of year are clean of sediment and algae (Archer and Tyus 1984; USFWS, unpublished data, Vernal, UT). There are substantial field and laboratory data suggesting that Colorado pikeminnow, and other ~ pikeminnow species, require clean cobble surfaces for successful adhesion of eggs (Patten and Rodman 1969; Hamman 1981). Hamman (1981) also noted hatching of Colorado pikeminnow larvae from cobble surfaces. Spring scouring, a gradual decrease in summer flows, and a concomitant decrease in sediment load minimize siltation of cobble bars. Recent studies (Harvey et al. 1993, Harvey and Mussetter It 1996) argue that spawning areas for Colorado pikeminnow in the Yampa River have a well-defined set of hydraulic and geomorphologic characteristics, and these characteristics seem to apply to spawning areas in the San Juan River (M. Harvey, pers. comm., 1997). 14 C