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944 OSMUNDSON ET AL. .? rL 7l 2 •: 00 ' MOAB o Go`0 m Lake Powell UTAH RULISON UPPER REACH 5 GRAND JUNCTION CAME' 6 ?© O ? ? 7y O' i I I o 2s so 7s 100 I I KILOMETERS I I 1 OLORADO FIGURE 1.-Map of the study area. The lower boundary is the Green River confluence. Upper and lower study reaches are separated by Westwater Canyon. Also marked are the upper ends of the study area on the Colorado and Gunnison rivers where diversion dams blocked upstream movement of fish. Dotted lines demarcate the boundaries of the seven study strata (numbered). Starred circles are locations of U.S. Fish and Wildlife Service thermographs; crossed circles are locations of U.S. Geological Survey gauging stations with thermographs. marily in more upstream reaches. The best spawn- ing habitat ostensibly occurs in middle to upper reaches, and long-distance drift provides a mech- anism for delivering emerging larvae to optimal nursery habitat, generally located far downstream (Tyus 1990). With young fish primarily in lower river reaches, higher concentrations of adults in upper reaches suggest that distribution of sub- adults or adults requires extensive upstream move- ments. To date, these movements have not been documented and factors associated with such movements are poorly understood. Our primary objectives were to describe the dis- tribution of subadults and adults in the Colorado River, document their dispersal patterns, and at- tempt to explain what motivates such dispersal. We hypothesized that upstream movement of young adults is a response to resource gradients. More specifically, we hypothesized that upstream dispersal results from individuals seeking foraging areas more likely to meet the changing food re- quirements of a growing piscivore, and we inves- tigated spatial variation in Colorado squawfish body condition and abundance of likely forage species. Longitudinal temperature gradients were also examined and related to the distribution of adults. Methods Study area.-The study area included the oc- cupied range of the Colorado squawfish in the Col- orado River, from the confluence with the Green River in Utah upstream to the Grand Valley di- version dam at Palisade, Colorado (the latter is a barrier to upstream movement; Figure 1). Also in- cluded was the lowermost 3.5 km of the Gunnison River, from the confluence with the Colorado River upstream to the Redlands diversion dam (also a barrier to upstream movement). Locations are in river kilometers (rkm) from the Green River con- fluence (rkm 0.0), converted from river miles as mapped by Belknap and Belknap (1974). The study area (Figure 1) was partitioned into a lower reach that extended from rkm 0.0 upstream to the lower end of Westwater Canyon (rkm 181.0) and an upper reach from the upper end of Westwater Canyon (rkm 200.0) upstream to the Grand Valley diversion dam (rkm 298.1). Westwater Canyon, a