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EXECUTIVE SUMMARY The Colorado River basin is home to several endangered warm-water fish, including the Colorado pikeminnow (Ptychocheilus lucius, formerly known as the Colorado squawfish). Demographic studies of the age and distribution of Colorado pikeminnow in the main stem of the Colorado River upstream of the Green River confluence indicate that juvenile pikeminnow are located primarily in the lower part of their potential range, while adults are located primarily in the upper part of their potential range. This distribution presumably reflects the availability of food resources and riverine habitats. However, outside of the reaches near Grand Junction, CO, not much is known about the influence of physical channel characteristics on the fish community structure of the Colorado River. The work described in this report is part of a 3-part study to determine how lengthwise variations in the physical characteristics of the Colorado River (width, depth, substrate sediment size, water quality, etc.) influence the production and availability of foods used by benthic invertebrates and native fishes. A better understanding of the links between physical and biological processes is necessary for determining effective strategies for species recovery. The present study encompasses approximately 300 kilometers of the Colorado River in western Colorado and eastern Utah. Field measurements characterizing the geomorphology of this segment of the river were obtained at closely spaced intervals from approximately Rulison, CO, to Potash, UT. These field measurements were augmented with streamflow and sediment data from four US Geological Survey gauging stations to evaluate temporal and spatial trends in suspended sediment loads. Separate sets of 1937 and 1995 aerial photographs of the reach between Rifle and DeBeque were also analyzed to determine the extent of historical changes in channel characteristics. Finally, detailed measurements were made in several short reaches to calibrate a hydraulic flow model, and derive relations for estimating discharges that mobilize the cobble and gravel bed material of the Colorado River. The results of the photogrammetric analysis of the Rifle-DeBeque reach indicate that the average width of the main channel of the Colorado River has not changed significantly in this reach in the last 60 years. In contrast, the area of islands has decreased by about 20%, while the area of side channels and backwaters has decreased by about 31 %. When proportioned over the total reach length of 58 km, the change in side-channel/backwater area amounts to decrease in average width of about 6 m. Comparison of results from this study with results from a previous study (Pitlick et al., 1999) indicates that the total area of side channels and backwaters present in the Rifle-DeBeque reach is roughly the same as the total area of side channels and backwaters lost historically in the 15- and 18-mile reaches near Grand Junction. It is not clear, however, that the potential habitat in the Rifle-DeBeque reach is as suitable in a biological sense as the reaches downstream. A recent analysis of water temperatures presented by Osmundson (1999) suggests that conditions favoring growth of Colorado pikeminnow are less suitable here than in reaches downstream. Nonetheless, it appears that adult pikeminnow would benefit from having access to the Rifle-DeBeque reach, particularly in low-water years when water temperatures are higher. The analysis of sediment transport measurements indicates that the annual suspended sediment load of the Colorado River increases markedly through the study area. In downstream order, the long- term average annual suspended sediment loads of the Colorado River are: 0.4 x 106 tons/yr at Glenwood Springs; 1.5 x 106 tons/yr at Cameo; 3.4 x 106 tons/yr at the Colorado-Utah state line; and 5.4 x 106 tons/yr at Cisco, UT. The available data indicate that there has been no appreciable change in the concentration or the grain size of suspended sediment carried by the Colorado River, however, average annual sediment loads have decreased significantly because the frequency of higher discharges has been reduced by reservoir operations. v