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INTRODUCTION <br />Alluvial and canyon-bound reaches of the Colorado River in western Colorado and <br />eastern Utah provide important habitat for three endangered fishes in the upper Colorado River <br />basin- the Colorado pikeminnow (Ptychocheilus-lucius), the razorback sucker (Xyrauchen- <br />texanus), and the humpback chub (Gila Cypha). Success in recovering these fishes will depend <br />in large part on the maintenance and improvement of existing habitats within several key reaches <br />of the Colorado River, including the 15-mile and 18-mile reaches near Grand Junction, Colorado. <br />Along with lower reaches of the Gunnison River, the 15- and 18-mile reaches represent the upper <br />limit of the current range of Colorado pikeminnow and razorback sucker on the mainstem of the <br />Colorado River; humpback chub are found in incised bedrock reaches further downstream (Black <br />Rocks and Weswater Canyon). The 15- and 18-mile reaches are characterized as having a mildly <br />sinuous channel pattern with varying amounts of complexity; bankfull depths average 2.5-3 m <br />and substrate grain sizes vary from granules to cobbles [Pitlick et al., 1999; Pitlick and Cress, <br />2002]. This combination of physical characteristics, together with light and nutrient availability, <br />provides for relatively high levels of primary and secondary production in comparison to reaches <br />further downstream, and a relative abundance of native prey fishes (flannelmouth sucker and <br />bluehead sucker) [Osmundson et al., 2002]. Presumably, it is the availability of habitat in the 15- <br />and 18-mile reaches, and the abundance of potential prey fishes, that draw Colorado pikeminnow <br />upstream as they mature (razorback sucker, which are currently very rare, historically used the <br />15- and 18-mile reaches for purposes similar to Colorado pikeminnow; however, the resource <br />and habitat requirements of this species is only partly understood). Further migration by either <br />species to habitats upstream of the 15-mile reach is presently limited by a series of low-head