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5HO KAEDING AND ZIMMERMAN TABLE 1.-Range of physicochemical characteristics measured in the Little Golorado River, the confluence, and the Colorado River. Number of obserY~ations (cross sections or chemical determinations) is given in parentheses. Reach Width (m) Mean depth (m) Maximum depth (m) Dissolved oxygen Conductivity (mg/liter) (µmho/cm, 25 C) Salinity (%oo) Turbidity (formazin units) pH Little Colorado LC 1 2-8(4) 4,161-5,161(3) 2.1-3.3(3) LC 2 18.0-351(5) 1.0-1.5(5) 1.4-2.6(5) 7-10(9) 3,972-6,01'1(9) 2.0-3.1(7) 12-16(2) 9,6-8.0(4) LC 3 29.3-50.0(4) 0.5-0.9(4) 1.0-1.2(4) 8-10(B) 3,973-5,012(9) 2.0-3.5(9) 0-7(4) 9.8-7.7(4) LC 4 20.1-27.4(4) 0.7-0.9(4) 1.0-1.7(4) 8-10(7) 3,548-6,018(6) 1.7-3.0(6) 2-7(4) 7.5-8.2(5) Confluence C 4 75.6-]32.0(9)a 4.9-9.1(6)a 61-13.7(6)° 9-11(12) 503-4,990(18) 0.1-3,1(17) 0-310(9) 8.1-8.5(5) Colorado C 1 812-205,6(9) 3.8-8.5(9) 4.6-15.2(9) 10-II(5) 567-1,160(6) 0.0-0.4(6) 0-10(5) 7.5-8.0(3) C 2 77.7-193.5(9) 4.0-8.3(9) 52-17.1(9) 9-10(6) 483-787(6) 0.1-0.4(6) 2-36(5) 9.1-7.8(2) C 3 51.1-182.8(9) 4.3-6.4(9) 4.6-11.9(9) 10-II(7) 397-787(7) 0.1-0.5(6) 0-5(3) 7.9-8.0(2) C 5 71.5-]90.6(9) 4.0-11.3(9) 4.9-18.6(9) 9-11(5) 436-920(5) 02(5) 0-10(5) SA(2) C 6 65.2-124.6(9) 3.8-6.5(9) 6.4-10.4(9) 9-11(5) 534-778(6) 0.1-0.4(6) 2-5(4) 7.5-7.8(2) C 7 83.2-]92.8(9) 3.4-7.1(8) 5.5-9.8(8) 9-11(6) 551-778(7) 0.1-0.5(7) 4-60(4) 7.9-8.0(2) a All these measurements were taken in the Colorado River Resulting mean values of C/f then were ex- amined for relationships with seasons, river reaches, and diel periods. Because humpback chubs differed greatly in vulnerability to cap- ture between rivers, statistical analyses of catch data were made within rivers only. Linear- regression analyses were used to test for rela- tionshipsbetweenmeanC/f forhumpbackchubs and individual sympatric species within rivers. These analyses were made within quantitative gear types, and between quantitative gears fished simultaneously. Results and Discussion Phy~sicochemistry~ The physicochemical characteristics of the reaches differed markedly between rivers, but little within rivers (Table 1). Dissolved oxygen concentration was near saturation except in up- stream areas of LC 1, where carbon dioxide concentrations were high in spring discharges. Equilibration of dissolved gases with the at- mosphere results in considerable precipitation of travertine (CaCOg) at LC 2 and lower LC 1 (Cooley et al. 1969), where a rocklike accu- mulation 8 mm thick was found on the ther- mograph after it had been in the river for 3.5 months. A particularly large travertine dam marked the boundary between LC 1 and LC 2, and LC 2-4 contained numerous travertine dams 0.1 to 2 m high. The height of the dams affected the depth and length of intervening pools, as indicated by the following percentages of the thalweg consisting of pool habitat: LG 2, 70%; LG 3, 54%; LC 4, 34%. Sodium chloride contributes greatly to the high specific conductance and salinity of the Little Colorado (Cooley et al. 1969). Because the volume of the Little Colorado is relatively small (about 5% that of the Colorado), the river has little effect on the physicochemical char- acteristics of the Colorado, although in flood the Little Colorado greatly increases turbidity in the Colorado (Cole and Kubly 1976). Only one turbidity measurement (from C 4) was tak- en during a flood in the Little Colorado. Mean water temperatures were about 9 C warmer in the Little Colorado (specifically at reach LC 4) than in the Colorado throughout the year (Fig. 2). Unlike river temperatures be- fore dam construction, Colorado River tem- peratures varied little annually within the study area. Relative Colorado River water-surface el- evation fluctuated 0.6 to 1.9 m within 5-day periods, whereas little short-term variation oc- curred before Glen Canyon Dam was construct- ed (see Dolan et al. 19'74). Age and Growth Scale annuli were useful indicators of age of Little Colorado River humpback chubs; annuli correlated directly with modes in length-fre- A d