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578 KAEDING AND ZIMMERMAN icek and Kramer 1969; Vanicek et al. 1970; Holden and Stalnaker 1975). The dam dis- charged hypolimnetic water from the reservoir and temperatures in the tailwater remained ]ow throughout the year. Lower tailwater temper- atures and the loss of lotic habitat in the res- ervoir presumably eliminated reproduction by some native fishes and led to their eventual ex- tinction in that region. Although comprehensive surveys of the fishes of the 400-km Colorado River reach that in- cludes the Grand Canyon did not begin until several years after the 1963 closure of Glen Canyon Dam upstream, results of these inves- tigations suggested that dam operation affected native fishes in much the same way as it affected them below Flaming Gorge Dam. Researchers in the Grand Canyon failed to collect the Col- orado squawfish Ptychocheilus lucius, razorback sucker Xyrauchen texanus, or bonytail Gila ele- gans, although these species probably once oc- curredthere. Afew humpback chubs Gila cypha were found, however. They were most abun- dant, and apparently reproduced, near the con- fluence with the Little Colorado River, a small though significant tributary (Minckley and Blinn 1976; Suttkus and Clemmer 1979; Minckley et al. 1980). In the present report, we describe the life history and ecology of the humpback chub near the confluence of the Little Colorado and Colorado rivers in the Grand Canyon. Methods Study Area and Sampling Scheme The study area included 32 km of the Colo- rado River, and 20 km of the Little Colorado River to Blue Spring, the farthest upstream source of perennial water for the Little Colo- rado within the Grand Canyon (Fig. 1). The rivers were divided into reaches about 5 km long, except the confluence (reach C 4) included both rivers and totaled 3 km. Sampling at LC 1, the most upstream reach of the Little Colo- rado, was stopped after intensive collecting ef- forts in May and July 1980 yielded no hump- back chubs. Before each sampling trip, we randomly se- lected one 0.5-1-km-long sampling site within each reach, using large-scale aerial photo- graphs. Seven quantitative sampling trips were made along the Little Colorado, and three semi- annualquantitative trips along the Colorado, in 2 years; however, only two Little Colorado ci ENLARGED AREA C 2 Arizona C3 >_ ~ LITTLE C~~ C4 ~H LC4 •R ~O LC 3 p O 9 CS LC2 0 U r . LC 1 cs Biue Spring North c7 o z kilometers FlcuxE 1.-Study area showing locations of sampling reaches in the Colorado River (C 1-7) and Little Col- orado River (LC 1-4). Arrows along the rivers denote directions of flow. reaches were sampled during some trips. Aerial photographs were used on the ground to ac- curately identify the locations of individual col- lection areas. Sampling was stratified by three diel periods: morning, afternoon, and night (darkness). Sampling at each site usually contin- ued for 2 days. Physicochemical and Biological Measurements River cross sections were measured manually in the Little Colorado and electronically in the Colorado. Chemical analyses of the water were performed in the field between 1000 and 1400 hours. Continuously recording thermometers were placed in the Little Colorado at two lo- cations, 5 and 13 km above the confluence of the rivers, and in the Colorado at the United States Geological Survey gauge 23 km below the study area. Temperatures at the gauge were representative of those in the study area; Cole and Kubly (1976) showed that water tempera- ture in the Colorado increases no more than A v