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<br />DOWNSTREAM EFFECTS OF RESERVOIR ON RIVER, COLORADO AND UTAH
<br />
<br />1017
<br />
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<br /> : ~66~ eOi'
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<br /> 100 1000 10,000 100.000 100 1000 10.000 100,000
<br />
<br />WATER DISCHARGE IFP/SI
<br />
<br />before October 1962 and 66 after October
<br />1962. Using these measurements, daily sediment
<br />transport rates were determined for 6 size
<br />fractions-<0.004 mm, 0.004-0.016 mm,
<br />0.016-0.0625 mm, 0.0625-0.125 mm, 0.125-
<br />0.250 mm, and 0.250-0.500 mm-aswell as
<br />for al\ sand-sized material and all material for
<br />the pre- and post-reservoir periods at the Green
<br />
<br />River at the Green River, Utah, gage. The trans-
<br />port rate of suspended sediment in 4 size
<br />fractions-O.OO4-O.016 mm, 0.0625-0.125 mm,
<br />sand-sized, and all material measured during the
<br />pre- and post-reservoir periods-are plotted in
<br />Figure 4 versus the associated water discharge.
<br />As shown previously for the Jensen gage, no
<br />appreciable difference in the suspended-sedi-
<br />
<br />Figure 3. Measured suspended-sediment
<br />transport rates for various-sized material as a
<br />function of water discharge at the Green
<br />River near Jensen, Utah, gage during the pre.
<br />reservoir (open triangles) and post-reservoir
<br />(solid dots) periods. A. 0.004-0.016 nunfrac-
<br />tion. B. 0.0625-0.125 nun fraction. C. Sand-
<br />size fraction. D. All sizes.
<br />
<br />ment transport rate of any particle size at a given
<br />discharge is apparent between the pre- and post-
<br />reservoir periods.
<br />For each sediment-sized fraction, a least-
<br />squares linear regression was fit to log-trans-
<br />formed values of water discharge and daily
<br />sediment transport rate measured at the Green
<br />River, Utah, gage during the pre- and post-
<br />reservoir periods. The regression equations de-
<br />termined for the pre- and post-reservoir periods
<br />were compared, using the F-test statistic. The
<br />regression equations and results of this analysis
<br />are summarized in Table 3. As was the case for
<br />values measured at the Jensen gage, the level of
<br />confidence at which no significant difference be-
<br />tween the pre- and post-reservoir periods is de-
<br />tected varies considerably among the several size
<br />fractions. In general, the confidence level im-
<br />proves with increasing particle size. For the
<br />sand-sized sediment, there is no significant dif-
<br />ference in the relation between transport rate
<br />and water discharge during the pre- and post-
<br />reservoir periods at the 90th percentile level. For
<br />all sediment sizes, there is no significant differ-
<br />ence in the pre- and post-reservoir transport rela-
<br />tions at the 95th percentile level of confidence.
<br />The confidence levels are slightly less than those
<br />determined for the Jensen gage. Nevertheless,
<br />the analysis indicates that there has been no sig-
<br />nificant change in the sediment transport rate at
<br />the Green River, Utah, gage as a result of Flam.
<br />ing Gorge Reservoir. As shown earlier, the in-
<br />flow of sediment to the reach upstream from the
<br />Green River, Utah, gage, exceeds the outflow by
<br />a considerable quantity, and the reach is accu-
<br />mulating nearly 2.0 x 106 tons/yr on an average.
<br />In spite of the large quantity of material that has
<br />accumulated in the reach upstream from this
<br />gage, the sediment transport relations bave re-
<br />mained remarkably constant.
<br />The theory of alluvial river channels (see
<br />Mackin, 1948; Leopold and Maddock, 1953)
<br />holds that hydraulic characteristics of a channel
<br />will adjust over a period of years to transport the
<br />quantity of sediment supplied with the available
<br />discharge. With regard to the current condition
<br />of the Green River in the vicinity of the Green
<br />River, Utah, gage, this principle indicates that
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