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METHODS Study Area The study area included that portion of the upper Colorado River mainstem presently and historically occupied by Colorado pikeminnow. The currently occupied section extends from the confluence with the Green River in Utah upstream to the Grand Valley Diversion Dam at Palisade, Colorado. The unoccupied section, where the Colorado pikeminnow has been extirpated, extends from Palisade upstream to Rifle, Colorado). In addition, to see how fish abundance in the nearby Gunnison River compared to that in the Colorado River, our study area also included a 19-km section of the Gunnison River (Figure 1). Colorado River locations are described in river kilometers (rk) from the Green River confluence (rk 0.0), converted from river-mile maps developed by the Colorado Division of Wildlife. Gunnison River locations are described in river kilometers from the confluence with the Colorado River. To evaluate longitudinal changes along the river continuum, a stratified random sampling approach was used. The study area for the mainstem Colorado River was first stratified on the basis of differences in average channel gradient and geology. Some of these strata were further subdivided into two strata based upon differences in geomorphology, resulting in a total of 15 strata. Two of these were omitted because they were transitional between alluvial and canyon types. The uppermost stratum (just downstream of Rifle, Colorado) was also omitted because of its similarity with the next downstream stratum. One stratum, Westwater Canyon, was omitted because large rapids and turbulent eddies required specialized sampling equipment and because few Colorado pikeminnow have been found there in past studies (T. Chart, UDWR, personal communication). The four omitted strata comprised 53 of 388 km (14%) of the general study area. The remaining study strata were numbered from 1 to 11 starting from the Green River confluence and proceeding upstream (Figure 1). The single Gunnison River stratum was designated Stratum 12. Using aerial photographs, each study stratum was divided into reaches consisting of one meander (riffle-run) sequence. Of these numbered reaches three were selected from each of the 12 strata using a random numbers table. Study reaches ranged from 1.0 to 3.1 km in length. Within Stratum 1, the three reaches were randomly selected from the uppermost 27- km section due to limited accessability below rk 76.3 and because previous strata-wide sampling indicated that habitat and electrofishing catch rates were relatively homogeneous throughout this stratum (Osmundson et al. 1998). Gradient, land form type, and river- kilometer locations of the study strata and study reaches are presented in Appendix Table 1), Relative Abundance Electrofishing catch rates were used as an index of relative abundance of main-channel species. We assumed that adult Colorado pikeminnow > 550 mm total length (TL) require large forage items as part of their diet (see Discussion) and therefore restricted our sampling