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i substrate, meandered to some extent within the floodplain, and were dominated by run and pool habitat. Canyon-bound river reaches were typically constrained, single-thread channels, that had a mix of riffles, runs, and pools, and a mix of substrate that ranged from sand to boulder-sized particles. River flows were highest in spring and early summer during snow-melt runoff and declined to base level by mid-summer (Fig. 2). The Green River Basin was divided into five main reaches: the Yampa and White rivers, and three reaches of the Green River. The Yampa River reach was 118-RK long and extended from RK 192 to 74; few Colorado pikeminnow have been documented upstream of that area (Holden and Stalnaker 1975). The canyon-bound lower reach of the Yampa River (74 RK) was excluded because few fish were thought to occur there outside of the spawning season, and because logistical constraints prevented extensive sampling there (Holden and Stalnaker 1975). We were unable to conduct supplemental sampling planned in autumn for most canyon reaches during this study because of low water levels. The sampled reach of the Yampa River flowed mostly through alluvial valleys but included Little Yampa Canyon in the upper portion of the study reach. The 5 RK-long Cross Mountain Canyon and the 3 RK-long Juniper Canyon were not sampled because they were inaccessible. The Yampa River reach was sampled by personnel from the Larval Fish Laboratory, Colorado State University. The middle Green River study reach was 143-RK long and extended from RK 539.4 (16.2 RK downstream of the confluence of the Yampa River confluence) downstream to just above Desolation Canyon at RK 396 (near the White River confluence). The reach was in an alluvial valley, with the exception of Split Mountain Canyon (RK 528.2 to 513.8), which was sampled infrequently. The 16.2 RK-long Whirlpool Canyon reach from the confluence of the Yampa River downstream to upper Island Park was also excluded from sampling. Most of the 15