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

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NAU Sand Bar Studies

Final Report

study sites to the 1996 Controlled Flood, In addition, the data indicate no response at the sites

following the August 10, Paria River flood. During the August 1997-Apri11998 period, the average

main channel bed thickness increased 0.33 m and 0.42 m in upper and lower Marble Canyon,

respectively (Fig, 7c). However, average eddy thickness remained relatively unchanged, suggesting

that eddies remained at a relatively full condition as compared to August 1997, Dam releases during

this period were moderately high [average daily mean of 538 ml/s (\9,000 fe/s)], The channel bed

thickness increase is possibly the result of the coarsest size fractions of the Paria River supplied sand

moving slowly downstream (compared to the finer sizes), In contrast, there is no indication that

deposition of Pari a-supplied sediment in eddies occurred during this period,

The total mass of sand stored at the study sites is illustrated in Fig. II for 1996-1998, During this

two-year period, the Paria River sand input was below normal in 1996 and above nonnal in 1997 (D,

Topping, written comm" 1999), On the basis of this mass sum, we conclude that within two years of

the 1996 Controlled Flood, sand storage at the sites had returned to levels comparable to those

measured before the flood, These data suggest that the low elevation areas recover from scour

following high flows, such as the 1996 Controlled Flood, over a period of several years, Recovery

results from a combination of intracanyon recycling (erosion and transfer of high-elevation sand back

to low elevation storage) and from tributary sand inputs. However, most of the recovery at the sites

occurred prior to sand inputs by the 1997 Paria River floods, This may indicate, as suggested by

Topping et al. (2000b), that the total volume of sand in low elevation eddy and channel storage in

Marble Canyon is small, compared to the amount of sediment supplied to it by the Paria River.

In order to prolong the residence time of tributary-supplied sediment in the system, a greater stage

increase is required to access high elevation areas available for deposition. Floods on the Paria River

do not raise main stem discharge high enough and for sufficient duration to result in channel margin

deposition above stage levels reached by nonnal dam releases, Timing higher flows to be coincident

with or shortly following the summer and fall sediment input season improves the likelihood that finer

sediments will be effectively conserved, especially within upstream reaches closest to the dam.

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