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<br />Measured and Predicted Velocity and Longitudinal Dispersion at Steady and Unsteady Flow,
<br />Colorado River, Glen Canyon Dam to Lake Mead
<br />
<br />rating curve - defined by recent current-meter mea-
<br />surements. Gaging stations with stage record and rat-
<br />ing curves were Colorado River below Glen Canyon
<br />Dam (09379910); Colorado River at Lees Ferry
<br />(09380000); Colorado River above the Little Colorado
<br />River near Desert View (09383100); Colorado River
<br />near Grand Canyon (09402500); Colorado River above
<br />National Canyon near Supai (09404120); and Col-
<br />orado River above Diamond Creek near Peach
<br />Springs (09404200) (Figure 1). The network of tempo-
<br />rary stage recorders provided stage information at
<br />intervals of about 8 kilometers. In addition, stage at
<br />sampling sites not near an existing gage was recorded
<br />during sampling for the unsteady-flow measurement
<br />with a portable gage that consisted of a submersible-
<br />pressure transducer and datalogger to provide addi-
<br />tional information on the timing of dye passage
<br />relative to the passage of the daily hydrograph.
<br />
<br />Results
<br />
<br />Steady Flow, Glen Canyon Reach. Results show
<br />that dispersion was proportionally much greater
<br />at the lowest flow than at the highest two flows
<br />
<br />measured (Figure 3 and Table 2). Velocity increased
<br />from 0.3 to 1.0 mls as discharge increased from 139 to
<br />651 m3/s. The lower bed slope in the Glen Canyon
<br />reach than in downstream subreaches (Table 1) may
<br />account for the fact that average velocity at a dis-
<br />charge of 425 m3/s is less in the Glen Canyon reach
<br />than in any of the subreaches of the Grand Canyon
<br />reach.
<br />The changes in unit-peak concentration, dye-cloud
<br />variance, and dye-cloud skewness with discharge all
<br />show that dispersion is much greater at the lowest
<br />flow than at the two highest flows (Figure 3). Dura-
<br />tion of the dye cloud past Lees Ferry, measured from
<br />the time of arrival of the dye to the time when a con-
<br />centration of 10 percent of the peak concentration was
<br />reached on the trailing edge of the dye cloud, also is
<br />proportionally much greater at the lowest flow than
<br />the two highest flows. Dye-cloud duration was 4.5
<br />hours at 651 m3/s, 6.5 hours at 425 m3/s, and 12.3
<br />hours at 139 m3/s. Quantitative measures of changes
<br />in spatia] characteristics of flow are not available, but
<br />qualitative information from aerial photographs and
<br />direct observation indicates that the increased
<br />dispersion at low flow results from an increase in sin-
<br />uosity and in longitudinal velocity gradients caused
<br />by the emergence of large cobble bars and riffles.
<br />
<br />Za:O 150 12.5 I , I
<br />Owm .
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<br /><:Joz 00 .
<br />0:0:",0 Zw 10.0
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<br />><Z -'0: .
<br />"'0 Ww
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<br />0 z
<br /> 0 I , 0 I , I
<br /> 0 200 400 600 800 0 200 400 600 800
<br /> DISCHARGE. IN CUBIC METERS PER SECOND
<br />
<br />Figure 3, Relation of Average Velocity, Dye-Cloud Skewness, Dye-Cloud Variance, and
<br />Unit-Peak Concentration to Discharge, Glen Canyon Reach.
<br />
<br />".
<br />'~r-
<br />
<br />271
<br />
<br />WATER RESOURCES BULLETIN
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