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EXECUTIVE SUMMARY Alluvial reaches of the Colorado River near Grand Junction, Colorado, provide important habitat for the endangered Colorado squawfish (Ptychocheilus lucius) and razorback sucker (Xyrauchen texanus).:.Populations of these native fishes have declined dramatically in the past several decades, a situation that is often attributed to the hydrological and ecological effects of upstream reservoir operations. This-report summarizes research done over the last five years to evaluate the importance of historical changes in streamflow and sediment loads on alluvial reaches of the Colorado River near Grand Junction. In addition, we describe recent changes in the geomorphology of the Colorado River and address the question of what can be done to improve existing fish habitats. Our analysis of streamflow data from USGS gauging stations in the upper Colorado River basin shows that peak and mean annual discharges of unregulated rivers have not changed significantly in the last 60 years. In contrast, the Colorado River and its major upper basin tributary, the Gunnison River, have experienced significant decreases in annual peak discharge because of reservoir operations. Since 1950, annual peak discharges of the Colorado River at Glenwood Springs have decreased by more than 40%; annual peak discharges of the Colorado River near Cameo have decreased by 29%; and annual peak discharges on the Gunnison River near Grand Junction have decreased by 38%. In addition, the annual hydrographs of both rivers have changed greatly due to reductions in peak flows and augmentation of base flows. Annual suspended sediment loads of the Colorado River and Gunnison River have likewise decreased over time. This was particularly apparent during a period from the early 1960s through the late 1970s when annual suspended sediment loads were 30 to 45% less than the long-term average. Analysis of aerial photographs indicates that between 1937 and 1993 the main channel of the Colorado River narrowed by an average of 20 in (-10-15% of its previous width), and about 1/4 of the area formed by side channels and backwaters has been lost. Observations of channel change during periods of above-average runoff from 1993 through 1995 indicate minor scour and fill of the main channel, but more significant scour and enlargement of side-channel and backwater habitats, and flushing of fine sediment from the bed. Modeled relations between discharge and dimensionless shear stress T* for the Colorado River indicate that the threshold for bed load transport (T* = 0.03) occurs at about half the bankfull discharge; these flows are exceeded about 30 days per year, on average. The bankfull discharge produces an average T* of 0.047 and is exceeded 7 days per year, on average. The width and depth of the main channel thus appear to be set by a discharge that produces an average boundary shear stress that is about -1.5 times the critical shear stress. A magnitude-frequency analysis of sediment transport indicates that a large proportion of the annual sediment load of the upper Colorado River is carried by moderate to high discharges. Under the present hydrologic regime, 65-78 % of the annual sediment load is carried by flows greater than about 1/2 the bankfull discharge. v1I