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<br />
<br />24
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<br />48
<br />
<br />ELAPSED TIME FROM START OF MODEL. IN HOURS
<br />
<br />72 96
<br />
<br />120
<br />
<br />144
<br />
<br />168
<br />
<br />Figure 10. Discharges Simulated to Represent Two Glen Canyon Dam
<br />Environmental Impact Statement Flow Alternatives.
<br />
<br />steady releases the highest (Figure 11). Solute-cloud
<br />duration shows a less systematic change from flow to
<br />flow. Over much of the Grand Canyon reach, little dif-
<br />ference in duration is shown by the results (Figure
<br />11). Solute clouds are estimated to be of shorter dura-
<br />tion at the upstream two sites and of longer duration
<br />at the downstream end of the reach for the high-fluc-
<br />tuating flow alternative than for steady releases (Fig-
<br />ure 11).
<br />
<br />DISCUSSION AND CONCLUSIONS
<br />
<br />Results indicate that unsteadiness offlow has little
<br />effect on flow velocity or longitudinal dispersion at the
<br />moderate mean discharge at which the Grand Canyon
<br />reach was measured in this study. Greater dye loss
<br />estimated for unsteady flow than for steady flow may
<br />be an indication that some water is temporarily
<br />stranded by decreasing stage during unsteady flow. In
<br />the Glen Canyon reach, average flow velocity varies
<br />linearly with mean discharge, but dispersion is much
<br />greater at the lowest of the three measured flows
<br />than at the two highest flows (Figure 3). Greater dis-
<br />persion may be caused by increased sinuosity and
<br />longitudinal-velocity gradients that accompany the
<br />emergence of large cobble bars and riffles at low flow
<br />(140 m3/s). Similar changes in channel geometry with
<br />discharge occur in some individual subreaches of the
<br />Grand Canyon study reach, and the low dispersion
<br />measured in the Grand Canyon reach at both steady
<br />
<br />WATER RESOURCES BULLETIN
<br />
<br />and unsteady flow may not be indicative of dispersion
<br />during flow releases with a low daily mean discharge.
<br />Results of this study have implications for manage-
<br />ment of dam releases to protect or enhance elements
<br />of the riparian ecosystem in the Grand Canyon. For
<br />example, the results show that the daily range of dam
<br />releases has little effect on those elements of the flu-
<br />vial system that move with the fluid, such as dis-
<br />solved constituents, clay-sized sediment, and fine
<br />organic material. Also, the results indicate that the
<br />rate of exchange between the eddies and the main
<br />downstream flow is high, because a slow rate of
<br />exchange would have caused greater longitudinal dis-
<br />persion than was observed. The daily range of dam
<br />releases has little effect on the rate of exchange
<br />between eddies and the main flow at the moderate
<br />daily mean discharge measured in this study. If zones
<br />are present that trap water for a significant length of
<br />time, then either their volume is small enough that
<br />they have no detectable effect on fluid transport in
<br />the main channel or they are sufficiently disconnected
<br />from the main flow that very little exchange takes
<br />place. Additional evidence of high exchange rate
<br />between the main channel and slower-moving areas
<br />along the banks is given by Maddox et al. (1987), who
<br />found that water temperature was higher than the
<br />main channel water temperature only in backwaters
<br />that were not directly connected to the main channel.
<br />The greater longitudinal dispersion measured at
<br />low flow (141 m3/s) in the Glen Canyon reach may be
<br />caused by a lower exchange rate between eddies and
<br />the main flow at low flow, but it appears unlikely
<br />
<br />278
<br />
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