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Last modified
7/14/2009 5:02:34 PM
Creation date
5/20/2009 10:25:48 AM
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UCREFRP
UCREFRP Catalog Number
8274
Author
Pitlick, J. and R. Cress.
Title
Longitudinal Trends in Channel Characteristics of the Colorado River and Implications for Food-Web Dynamics.
USFW Year
2000.
USFW - Doc Type
Recovery Implementation Program Project 48-C,
Copyright Material
NO
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CONCLUSIONS <br />1) Comparative analysis of 1937 and 1995 aerial photographs of the segment of the Colorado River <br />between Rifle and DeBeque, CO, indicates that the average width of the main channel has not <br />changed significantly in this reach in the last 60 years. In contrast, there have been relatively major <br />changes in the area of islands and side channels. We estimate that between 1937 and 1995, the area <br />of islands decreased by 20%, while the area of side channels and backwaters decreased by 31 %. <br />When proportioned over a reach length of 58 km, the change in side-channel and backwater area <br />amounts to a decrease in average width of about 6 m. <br />2) Although there has been a reduction in the amount of in-channel habitat in the Rifle-DeBeque <br />reach, a significant amount of potential habitat still exists. Comparison of results from the present <br />study with results from a previous study (Pitlick et al., 1999) indicates that the total area of side <br />channels and backwaters remaining in the Rifle-DeBeque reach is roughly the same as the total area <br />of side channels and backwaters lost historically in the 15- and 18-miles reaches. However, it is not <br />clear that the potential habitat in the Rifle-DeBeque reach is as suitable in a biological sense as <br />some reaches downstream; Osmundson's (1999) analysis of water temperatures here suggests that <br />conditions favoring pikeminnow growth are suboptimal (< 25° C) much of time. Nonetheless, adult <br />Colorado pikeminnow would probably benefit from having access to the Rifle-DeBeque reach, <br />particularly in low-water years when water temperatures are higher. <br />3) Our analysis of discharge and suspended sediment data from four USGS gauging stations on <br />the main stem of the Colorado River indicates that annual suspended sediment loads increase by a <br />factor of almost four through the study area. In downstream order, the long-term average annual <br />suspended sediment load of the Colorado River increases from 0.4 x 106 tons/yr at Glenwood <br />Springs, CO (upstream of the study area), to 1.5 x 106 tons/yr at Cameo, to 3.4 x 106 tons/yr at the <br />Colorado-Utah state line, to 5.4 x 106 tons/yr at Cisco, UT, in the lower part of the study area. <br />4) Reservoirs and flow diversions have had system-wide impacts on the sediment-transport regime <br />of the Colorado River. Major changes in sediment transport capacity appear to have occurred in the <br />late 1950s and early 1960s. Comparison of the periods 1934-1958 with 1959-1997 indicates that <br />annual suspended sediment loads were 25-35% lower in the more recent period than in the earlier <br />period. Changes in the frequency of high flows are largely responsible for the reduction in annual <br />sediment loads in the latter period. <br />5) Field measurements of the existing characteristics of the Colorado River indicate that the channel <br />gradient, substrate sediment size, bankfull width and bankfull depth change systematically down- <br />stream. The channel gradient decreases rapidly in comparison to the grain size, and the bankfull <br />depth increases downstream more rapidly than the bankfull width. Over the 300-km length of the <br />study area the bankfull depth increases by 100% (approximately doubling) while the bankfull width <br />increases by only 30%. These trends have important implications for primary productivity because, <br />in downstream reaches, conditions favorable for primary productivity may exist only where the flow <br />is relatively shallow, i.e. along the very edge of the channel or over riffles. <br />6) The longitudinal changes in channel depth, slope and grain size occur in such a way that the <br />bankfull dimensionless shear stress, z, is roughly constant through the gravel-bedded portions of <br />the study area from Rulison, CO, to Professor Valley, UT. This result has important implications <br />for sediment transport because it indicates that the width and depth of Colorado River are adjusted <br />such that the threshold for coarse-sediment transport is roughly the same from reach to reach. <br />41
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