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OBJECTIVE AND HYPOTHESIS The major objective of this specific study defines the scope of this portion of the integrated investigation which evaluated the trophic structure in the upper Colorado and Gunnison rivers. One hypothesis was tested in the process of meeting this objective. Objective 1: Determine longitudinal patterns, riverine physical attributes (habitat distribution, substrate particle size, depth of interstitial voids, light transmission, and sediment organic content) and biological (periphyton and invertebrate biomass); quantify relationships between riverine physical and biological attributes throughout the upper Colorado River. Hypothesis H1: Primary and secondary productivity varies longitudinally and is correlated with physical attributes, such as sediment organic content, light transmission, gradient, substrate particle size, and/or depth of substrate interstitial voids. METHODS Research Approach The river continuum concept (Vannote et al. 1980) was central to the formulation and fulfillment of objectives in this research project. This concept proposes that the structure and function of river communities are in direct response to physical conditions within the river that change from the headwaters to the mouth. It contends that the biological community responds predictably to physical habitat characteristics which are largely regulated by fluvial geomorphic processes. To assess changes in the biological community in response to different hydrographs requires an understanding of these processes and relationships. Minimal work has been conducted within the Colorado River system which relates the biological systems to the physical environment. Such research was recently conducted in the lower Colorado River within the Grand Canyon to assess effects related to operations of Glen Canyon Dam (Angradi et al. 1992) under recommendation of the National Academy of Science (NRC 1987). Within the Green River portion of the upper Colorado River basin, the river continuum concept was applied to characterize the White River ecosystem to provide a baseline for assessing effects of oil-shale development (ERI 1982). This study found that numbers of macroinvertebrates and native fish generally declined downstream and corresponded to gradient induced changes in substrate particle size and general habitat type. Also, the biomass of periphyton and macroinvertebrates were related to the dominant substrate particle size, and invertebrate collectors of coarse particulate organic matter increased in areas of high leaf litter input from cottonwood bottoms. Annear (1980) characterized the ecosystems of the lower Yampa River and Green River below Flaming Gorge Dam. 3