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EXECUTIVE SUMMARY A program designed to monitor embeddedness of gravel and cobble substrates in the upper Colorado River was initiated. Twenty measurements were taken at each of 15 sites (eight in the 15-mile reach; seven in the 18-mile reach) on seven dates in 1996 (three during runoff, four during base flow) and five dates in 1997 (three during runoff, two during base flow). Rock sizes were generally larger lower in the channel at base flow than higher in the channel margin inundated during runoff. Additionally, average rock size was slightly larger in the 18- mile reach than in the 15-mile reach, but only at base-flow locations. Spring runoff flows in both the 15- and 18-mile reaches were sufficient in both years to mobilize the bed in many areas. In addition, runoff flows during 1995 (prior to our sampling) produced widespread mobilization of the bed. Average rock sizes changed significantly between 1996 and 1997 at several of our sampling sites. Mean depth-to-embeddedness (distance from the top of rocks in the surface layer down to the point where rocks are embedded in fine sediment) ranged from 80 to 171 nun; the adjusted mean of all samples was 125 mm in 1996 and 123 mm in 1997. The mean number of `free rocks' (those making up the layer above the level of embeddedness) ranged from 1.1 to 4.3; the adjusted mean was 1.9 rocks in both years. Riffles had slightly greater depths to embeddedness than did runs. For other comparisons, depths to embeddedness were found to be similar between 15- and 18-mile reaches, between 1996 and 1997, and between runoff and base-flow periods. Rates of sedimentation were low during summer and fall following runoff, and depth-to-embeddedness remained fairly constant through the base-flow period. The two years of initial sampling represent a period of time when both reaches were freshly cleaned of fine sediment and our data should provide a good baseline for future monitoring. vi