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1734 D. B. OSMUNDSON ET AL. Ecological Applications Vol. 12, No. 6 30 25 Z 20 N c N 15 U c m 10 U N W Stratum Fto. 8. Mean recurrence intervals (bars) of discharges necessary to produce widespread streambed mobilization in strata 6-11 during recent (1966-2000) and preregulation (1908-1942) periods. Lines above bars indicate the maximum recurrence intervals. downstream decline of biomass in riffles, even though fine sediment in riffles was fairly consistent, suggests involvement of additional factors. Also, the highly sig- nificant multiple regressions of chlorophyll a and in- vertebrates with both PC score and river kilometer in- dicated that both cleanness of the bed and factors as- sociated with longitudinal location were important. Possible factors varying longitudinally include water clarity, water velocity, nutrients, water quality, water temperature, and detritus. Our measurements of tur- bidity, although very limited, suggested a downstream decrease in light penetration which could affect pri- mary productivity. Although not measured, mean water velocity may decline downstream due to the general attenuation of gradient. If so, surficial sediment de- position, expected to be more prevalent in areas with low base-flow velocities might explain the weak cor- relation found between water velocity and chlorophyll a and invertebrate biomass. Concentrations of nitrogen and phosphorus tended to increase downstream, and there was no correspondence between higher nutrient concentration and higher biomass. Lower nutrients in the upper river may have reflected greater uptake in the more productive strata. Although oxygen was not monitored, bed samples provided no indication of an- aerobic conditions. Contaminate concentrations were not evaluated, but we would expect levels to be highest in upstream strata where urban and agricultural areas border the river (elevated selenium levels have been reported in off-channel habitats within strata 8 and 9 [Osmundson et al. 2000]). Water temperatures increase with distance downstream (Osmundson et al. 1998), but did not reach levels that would inhibit primary or secondary production. Detritus declined downstream and the weak correlation with invertebrate biomass suggests a possible food linkage, but does not explain higher standing crops of periphyton. The downstream decline of detritus probably resulted from correspond- ing periphyton declines (an instream source) and the distance from upstream terrestrial sources (i.e., from alluvial reaches where riparian zones were most ex- tensive). While difficult to ascertain from this study, it appears that water clarity, water velocity, and detritus were factors that declined downstream and may have, in addition to bed sediment effects, influenced standing crops of primary producers and invertebrate consum- ers. Riffles were more productive than runs primarily be- cause riffle substrates generally contained less fine sed- iment than adjacent run substrates. In addition, riffles generally had higher velocities and contained more de- tritus than runs, variables that were weakly correlated with chlorophyll a and invertebrate biomass. Detritus, a food for many invertebrates, may have been signif- icantly higher in riffles because of more periphyton biomass and better detritus-collecting properties of "clean" substrate. Also, increased velocities in riffles may have benefited suspension feeders by increasing encounter rates with drifting detritus. The downstream decrease in surface area of riffles likely contributed to downstream declines in fish bio- mass. Runs were found to be increasingly less pro- ductive than riffles downstream, increasing the impor- tance of riffles in supporting stratum-wide fish num- bers, yet the surface area of riffles decreased down- stream as run area increased. Hence, downstream fish not only had less total periphyton and invertebrate bio- mass available per unit area, but also less total area of the more productive habitat type. Factors related to the longitudinal patterns of biota in the upper Colorado River were generally consistent with observations reported by various stream investi- gators. Increased sedimentation has been shown to re- duce insect diversity, density, and species richness in streams (Chutter 1969, Bjornn et al. 1977, Lenat et al. 1981). Lab studies have demonstrated macroinverte- brate preference for substrates with unembedded cob- ble over those with half-embedded cobble; and cobble completely embedded in sand is unacceptable to most species (Brusven and Prather 1974). Factors underlying these relationships are fairly well established: rock-sur- faces provide attachment sites for periphyton; algal fil- aments in turn provide food and effective microenvi- ronments for some invertebrate species (Brusven and Prather 1974); crevices among coarse substrate parti- cles shelter invertebrates and collect detritus (Rabeni and Minshall 1977). Schlosser (1982) reported a pos- itive influence of increased algal and invertebrate pro- duction on growth, reproductive success, and recruit- ment of stream-dwelling fish. Berkman and Rabeni (1987) found that feeding guilds of stream fishes most 1 2 3 4 5 6 7 8 9 10 11