WSP09464

• File Number: 8220.101.10.D
• Description: Glen Canyon Dam/Lake Powell
• State: AZ
• Basin: Colorado Mainstem
• Date: 5/24/2000
• Author: Hazel et al.
• Title: Sand Deposition in the Colorado River Ecosystem from Flooding of the Paria River and the Effects of the Nov 1997 Glen Canyon Dam Test Flow
• Water Supply Pro - Doc Type: Report/Study

NAU Sand Bar Studies

Final Report

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DISCUSSION AND CONCLUSIONS

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In this report we document the initial distribution of sand added to the 98-km long Marble
Canyon reach of the Colorado River ecosystem during a two-month period, During August-September
1997, the Paria River contributed nearly twice the mean-annual Paria River sand input. We also
examine the downstream redistribution of those sand inputs and evaluate the effects of the 1997 Test
Flow at redistributing sand to higher elevation locations in eddies, We compare field surveys of long-
term storage sites to modeled estimates of the Paria River sand inputs and measurements of sediment
export, The comparison of sediment storage changes using different methods is possible because the
measurement error associated with topographic surveys is very small relative to the volumetric
changes, and is considerably less than the errors associated with the other. methods: sediment transport
calculations and suspended-sediment measurements [i,e" uncertainties of 5-20% of the mean
(Appendix A in Topping et ai" 2000a)].
The series of short duration, large floods on the Paria River in late summer 1997, supplied the
Colorado River ecosystem with approximately 770,000 m3 (2,O:t 0.4 million Mg) of sand, Detailed
channel surveys of the 3-km reach at the head of Marble Canyon, immediately downstream from the
Paria River, show that approximately 24 to 36% of the sand input from these floods was immediately
deposited in this 3-km reach, Large increases in suspended-sediment transport at the lower end of
Marble Canyon were measured within 1-2 days of these Paria River floods, suggesting that a
measurable fraction of the supplied sediment was never deposited on the bed and was carried in
suspension through Marble Canyon at or near the speed of the daily discharge release, The finer grain
sizes travel downstream faster than the coarser sizes following tributary input (Topping et ai" 2000b),
Approximately 56,000 m3 (0,15 :t 0,03 million Mg) of sand were exported from Marble Canyon during
the 25-day sampling period in August-September, an amount equivalent to about 7% of the sand
supplied by the Paria River, Following the last Paria River flood on September 26, approximately
50% of the flood deposition in the 3-km reach was eroded during the 37 days prior to release of the
1997 Test Flow,

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Because there were no measurements of suspended sediment transport at the lower Marble Canyon

gage during the interval of time between the August-September USGS sampling program and the 1997

Test Flow, we could not detennine the amount of Paria River sand inputs that were still retained in

Marble Canyon prior to the test flow or how much of the Paria River input still remained upstream

from the lower Marble Canyon gage following the test flow, However, the sand-transport rates at the

lower Marble Canyon gage during the 1997 Test Flow were twice that observed during the 1996

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