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In this study, the river continuum concept was used to quantitatively describe the longitudinal distribution ofperiphyton, macroinvertebrates and fishes, as well as key physical parameters. Local and regional deviations that result from influences of geology, riparian conditions, tributaries, and localized nutrient inputs to the system (Minshall et al. 1983) were also considered. The major objective was addressed by quantifying the longitudinal distribution of important riverine physical habitat attributes (habitat distribution, substrate particle size, depth of interstitial voids, light transmission, and sediment organic content) and the aquatic biology (periphyton and macroinvertebrate biomass) during baseflow prior to runoff and in late summer. These distributions of physical and biological factors were compared to determine whether relationships exist. Hypothesis H1 will be rejected if no relationships are found between measures of primary and secondary trophic level parameters and physical attributes of the riverine system (sediment organic content, light transmission, gradient, substrate particle size, depth of substrate interstitial voids), as well as site locations (river kilometer and gradient). Study Area This study was conducted on the mainstem Colorado River from the Green River confluence to Rifle, Colorado (RK 0-386), and lower Gunnison River (RK 0-92). This section of the Colorado River encompasses a 2nd, 3rd, and 4th order stream. It represents that portion of the Colorado River where temperature, gradient, and P/R ratio (Vannote et al. 1980) are rapidly changing along the river continuum. This includes the entire range of Colorado pikeminnow in the Colorado River above the Green River confluence (RK 0), and in the Gunnison River. The study reach also includes a portion of the Colorado River (RK 303-386) which was historically a pikeminnow habitat, but is now inaccessible due to the impassible Price Stub dam. UTAH MORAD0 11 10 8 9 6 s a 2 3 N xo.e C 12 STUDY REACHES WITHIN cataract THE COLORADO RIVER Canyon STUDY AREA Lake Powell Figure 1. Eleven study strata on Colorado River and one study stratum on Gunnison River identified for sampling in this project. Divisions in strata were based upon differences in geology, geomorphology and gradient. The upper Colorado River was divided into eleven strata based on differences in channel gradient and geology (Figure 1). The Westwater reach was not sampled because of the need for specialized sampling equipment in this rapid section of the Colorado River. The Gunnison River section (RK 54) was sampled. For each stratum, the river was divided into reaches of repeatable riffle/run sequences (0.97 to 3.1 kilometers in length) with boundaries demarcated by natural breaks in river morphometry. Three of these reaches were randomly selected in each 4