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PITLICK AND CRESS: DOWNSTREAM CHANGES IN CHANNEL GEOMETRY 34 - 5 1600 ,. ... .._ 15001 1400 I' m i 1300 w ,. 12001 1100 -- RD ------ ? 6G 151N- _. tHNd... (------ Rki°. ?_ PINY ? ... CK°, 017 4 •' q8': hiY 350 300 250 240 150 100 Distance Above Green River Confluence (km) Figure 3. Longitudinal profile of the Colorado River between approximately Rifle, Colorado, and Moab, Utah. The shaded lines indicate boundaries of reaches listed in Table 1. DeBeque Canyon, Westwater Canyon and Professor Valley. The first of these is caused by a series of three low-head diversion dams which divert water into the Grand Junction area. These dams alter the profile but do not reflect natural processes. The breaks in slope through Westwater Canyon and Professor Valley are related to local geologic and tectonic processes. The segments in between these points define smooth profiles, which are not strongly influenced by transitions in reach type (alluvial to quasi-alluvial) or by junctions with tributaries. The segment between DeBeque Canyon (rkm 300) and Westwater Canyon (rkm 205) for example, includes a major tributary, the Gunnison River at rkm 274, and a transition in reach type at rkm 245. Neither of these features appears to affect the profile in a strong way. Elsewhere we observe similar trends, indicating that tran- sitions between hard and soft sedimentary rocks have little influence on the overall form of the longitudinal profile. Similar to Rice and Church [2001 ], we find that the concave segments of the profile are better fit by quadratic functions than exponential functions. 4.2. Bed Material [16] The bed material of this segment of the Colorado River grades from cobbles and large gravels in the upper reaches to medium gravels in the lower reaches. Plots of specific percentiles of the surface grain-size distributions (D84, D50i and D76) define a weak trend of downstream fining (Figure 4). Locally high values in two of the lower reaches (Professor Valley and Big Bend) skew the data somewhat, but even with these values excluded, the surface sediment fines at relatively slow rates. The trend lines shown in Figure 4 (and subsequent figures) are fitted exponential relations, In y = In a + bx, where In a is the y intercept, b is the slope of the line, and x is the distance, measured upstream with respect to the Green River (rkm 0). In this case the coefficient a represents the grain size atx= 0, and b represents the rate of downstream fining. These values are listed in Table 3, along with relevant parameters for the subsurface sediment, channel morphology and shear stress. Comparison of the values of b for the surface percentiles indicates that large and small sizes fine down- stream at about the same rate (Table 3). In comparison to trends reported in other studies [Knighton, 1998; Rice, 1999; Gomez et al., 2001], the rate of surface-fining in. the Colorado River is relatively weak. In this case, surface particle sizes change slowly downstream because coarse material is continually supplied from local sources such as ephemeral tributaries, terraces, and valley side-slopes. Input from these sources is not large enough to overwhelm downstream trends, but apparently sufficient to replenish coarse material worn down by abrasion. [17] A close inspection of the data in Figure 4 suggests that the rate of downstream fining within several individual segments is higher than the overall trend. Stronger fining is apparent in the reach between DeBeque Canyon and West- water Canyon (rkm 300-200), and in the reach below Professor Valley (rkm 130-100). To evaluate the impor- tance of these differences, we separated the data into four segments bounded by the breaks in slope discussed pre- viously. For each segment we derived separate exponential relations for the surface D 0 (Table 4) (relations for the bank-full width, depth, shear stress and Shields stress were also derived, but, for the moment, we focus only on the downstream trends in surface D50). The results listed in 300 E 33 1 D ....,.. .-..... 400 350 300 2507 200 150 100 Distance (km) Figure 4. Downstream trends in percentiles of the surface (pavement) grain size distribution.