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It is unknown whether spawning historically occurred in the Palisade-to-Rifle reaches prior to the construction of the diversion dams. De Beque Canyon, upstream of the most upstream diversion dam, has numerous cobble-bar islands and side channels that may contain suitable sites for Colorado pikeminnow spawning (Anderson 1997). However, duration of temperatures in excess of 18 and 20°C, is short in years of high, extended runoff and timing of such temperatures is delayed compared to more downstream reaches (Osmundson 2000). Until Colorado pikeminnow can be reestablished in this area, and the degree to which they use this reach for spawning can be assessed, its potential as spawning habitat should be assumed. However, the suitability of potential spawning sites in alluvial reaches upstream of De Beque Canyon, also identified by Anderson (1997), is expected to progressively decline with decreasing upstream temperatures (see Osmundson et al. 1998). Reproductive success of Colorado pikeminnow is strongly influenced by the flow regime. Results from larval drift studies in the Colorado River during 1992-1996 indicated that highest larval production (drift densities) occurred in years with moderate (1996) to high (1995) spring flows, and lowest larval production occurred in years with low (1992 and 1994) spring flows (Anderson 1999, Trammell and Chart 1999a). However, high larval production alone does not necessarily result in high numbers of YOY in fall. McAda and Ryel (1999), using 15 years of fall YOY data and USGS flow records (1982-1996), found that antecedent flows were just as important in predicting YOY density in fall as were flows that occurred in the year of reproduction. Highest densities of YOY occurred in years that had high peak flows (>50,000 cfs at the state line USGS gauge) in the previous year and moderately high flows (30,000-40,000 cfs) in the year when the young fish were produced. Although high numbers of larvae are produced in years of very high runoff, such years are also characterized by high flows extending into August, which appear to have a negative effect on YOY numbers in fall. Larvae in such years are produced late and backwaters are fewer in number. McAda and Ryel (1999) suggested that larvae are carried downstream out of the nursery area at such times and are perhaps lost to nonnative fish predation in Lake Powell. However, although exceptionally high water in wet years result in low fall YOY numbers, it is important in setting conditions for successful reproduction in the following year, so long as the following year has moderately high spring flows followed 17