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It is difficult to predict the success that Colorado pikeminnow might have in spawn- ing or in establishing home ranges upstream of the 15-mile reach. However, from what we currently know about the temperature re- gime of the river and the temperature re- quirements of this species, the Grand Valley appears to be located near the upstream limits of the optimal range for this popula- tion. Thus, the importance of the 15-mile reach will not be diminished by the reestab- lishment of access to more upstream reaches. Clearly, providing passage to reaches upstream of Palisade should be pursued as part of recovery efforts for the razorback sucker and Colorado pikeminnow. Razor- back sucker persisted in this reach until recent times suggesting that its thermal requirements may be slightly lower than those for Colorado pikeminnow. Also, razorback sucker typically exploit warm, off- channel ponds and flooded areas. This be- havior may serve to offset temperature limi- tations of the main channel allowing the razorback sucker population to extend up- stream farther than the population of Colo- rado pikeminnow. However, restoring pas- sage is also anticipated to extend the range of year-round use by Colorado pikeminnow upstream an estimated 14 miles into canyon habitat downstream of De Beque. This will provide a much-needed addition to the lim- ited amount of adult habitat currently avail- able to this population. Also, the more up- stream De Beque-to-Rifle reach, though per- haps too cool for year-round use, might serve Colorado pikeminnow as a seasonal feeding area during the warm months of late summer (Osmundson 1999a). However, this more upstream reach should not be viewed as a substitute for the high quality habitat found within the warmer, alluvial reaches of the Grand Valley. Nevertheless, all sections of former range in the upper Colorado River are important and must be restored if the long-term viability of these endangered populations is to be assured. SUMMARY AND CONCLUSIONS During the course of their lives, indi- vidual Colorado pikeminnow may utilize the entire 185 miles of the upper Colorado River to meet the demands oftheir changing life history needs. Habitat selection in- volves tradeoffs between the relative avail- ability of various resources. More optimum thermal regimes for Colorado pikeminnow and razorback sucker are located in lower portions of the river, but substrates are unsuitable for spawning and food availabil- ity for adults is low. Further upstream, steeper gradient and local geological condi- tions produce coarser substrate and diverse channel morphology resulting in suitable physical conditions for spawning. In addi- tion, greater nutrient input and light pene- tration results in greater food production in upstream reaches, particularly in alluvial valleys. However, for warm-water species, like Colorado pikeminnow and razorback sucker, the further upstream an individual ventures, the cooler the thermal regime becomes, and the more the potential for growth declines. The greatest concentra- tions of adult Colorado pikeminnow are found in the Grand Valley, presumably because the benefit of high food availability outweighs the cost of a cooler thermal regime. Further upstream, food availability remains high but temperatures decline until food availability can no longer offset the negative effects of low temperatures. Thus, 12