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Longitudinal Trends in Channel Characteristics Our description of downstream trends in the channel characteristics of the Colorado River begins here with a discussion of the relation between channel gradient and particle size. These two variables are closely related, and it has been shown in previous studies that, in the absence of bedrock or structural controls, grain size and bed elevation decrease exponentially downstream (e.g. Bradley et al., 1972). Figure 16 shows a plot of the longitudinal profile of the Colorado River from approximately Rulison, CO to Moab, UT. When plotted at this scale the profile appears to be relatively smooth, except for a steep section through Westwater Canyon (RKM 180-190). Although it is not particularly obvious, the y-axis is scaled in logarithmic units. Plotting the data this way illustrates the fact that the longitudinal profile deviates slightly from the expected exponential trend. A straight line exponential fit through all data points gives an average gradient of 0.0010, but the profile clearly deviates from the average slope at the upper and lower ends. Colorado River, Rulison, CO, to Moab, UT 1600 6 1500 c 1400 ?a 1300 W 1200 Distance Upstream From Green River Confluence (km) Figure 16. Longitudinal profile of the Colorado River from Rulison, CO, to Moab, UT. The y-axis is scale in logarithmic units to show that the profile deviates from an exponential trend given by the inset equation. The inflections in the profile at RKM 200 and RKM 140 correspond to reaches through Westwater Canyon and Professor Valley, respectively. Detailed plots of channel gradient derived from GPS measurements in specific subreaches show that there can be subtle inflections in the river's profile (Fig. 17). These inflections are not always caused by transitions in bedrock lithology, as might be expected. In the reach between Rifle and DeBeque Canyon, for example, there is a slight decrease in slope at about RKM 340 (Fig. 17a). This point is 10 km upstream of the nearest tributary, Roan Creek, and more than 15 km upstream of DeBeque Canyon, so it is unlikely that these features have any influence on the profile at that point. In contrast, there is no change in channel gradient where the Colorado River enters DeBeque Canyon and encounters more resistant bedrock (Fig. 17a). Likewise, there is no change in gradient in the transition between the 18-mile reach and the Ruby-Horsethief Canyon reach. In contrast, measurements between Cisco landing and Moab, UT show that the profile becomes much steeper through Professor Valley (Fig. 17b). As mentioned earlier, the inflection in the profile at this point is due to neogene uplift and deformation caused by vertical movement of salt. 27 350 300 250 200 150 100