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6 • of its characteristics in a laboratory flume study. O'Brien (1984) concluded that incipient motion of the cobble bed is required to clzan cobbles for spawning and estimated that this takes dischazges of about 21.00 cfs. However. Harvey et al.(1993) concluded that since flows required for incipient motion of bed material are raze (20 } eaz return period event) and spawT~ing occurs annually, another process must be cleaning the cobbles. Their study found that in Yarnpa Canyon recessional flows routinely dissect gravel bars and thereby produce tertiary bazs of clean cobble at the base of the riffles. These tertiary bars aze used by Colorado pikemittnow for spawning. Harvey et al. (1993) recognizes the importance of high magnitude, low frequency dischazges for the formation and maintenance of midchannel bars. Dissection of bars without redeposition by high magnitude flows would lead to conditions where spawning habitat is no longer available (Harvey et al. 1993). It is unknown whether tertiary bazs used for Colorado pikeminnow spawning in Yampa Canyon are available in the 15-mile reach of the Colorado River. There, significant motion of bed material occurs at neaz bankfull discharge of 22,000 cfs (Van Steeter 1996). These flows occur on average once in 4 yeazs. Van Steeter (1996) concludes that flows of this magnitude are important because they generally remove fine sediment from the gravel matrix which maintains the invertebrate community and cleans spawning substrate. Although the location of spawning areas in the Colorado River is not well defined, the presence • of larvae downstream of the Walker Wildlife Area, in the Loma to Black Rocks reach and near the confluence of the Dolores River, demonstrates that spawning does occur. Osmundson and . Kaeding (1989 and 1991) reported that water temperatures in the Colorado River were suitable for spawning in the Grand Junction area. In 1986, a year of high runoff, suitable temperatures for spawning (20° C) occurred in the first week of August. In 1989, a year of low runoff, the mean temperature reached 20° C during the last week of June. Miller et al. (1982) and Archer et al. (1986) demonstrated that Colorado pikeminnow often migrate considerable distances to spawn in the Green and Yampa Rivers, and similaz movement has been noted in the main stem Colorado River. Miller et al. (1982) concluded from collections of larvae and young-of--year below known spawning sites that there is a downstream drift of larval Colorado pikeminnow following hatching: Extensive studies in the Yampa and upper Green Rivers have demonstrated downstream distribution of young Colorado pikeminnow from known spawning azeas (Archer et al. 1986; Haynes et al. 1985). Miller et al. (1982) also found that young-of--year Colorado pikeminnow, from late summer through fall, preferred natural backwater azeas of zero velocity and less than 1.5-foot depth over a silt substrate. Juvenile Colorado pikeminnow habitat preferences aze similar to that of young-of-yeaz fish, but they appear to be mobile and more tolerant of lotic conditions away from the sheltered backwater environment. Information on radio-tagged adult Colorado pikeminnow during fall suggests that fish seek out deepwater areas in the Colorado River (Miller et al. 1982), as do many other riverine species. •