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 Studh!s

Final Report

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CHANNEL AND EDDY STORED SEDIMENT IN MARBLE CANYON

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Sand transported during the 1997 Test Flow must have come from locations on the channel bed,

low elevation parts of eddies, or other areas along the inundated channel margin, Unfortunately,

hydrographic surveys were not conducted downstream of the 3-km study reach in November 1997, and

an examination of subaqueous change at the downstream study sites was not possible owing to

restrictions on use of motorized craft during the fall season. Nonetheless, estimates of bed thickness

and total sand mass at the study sites in Marble Canyon for other time periods provide valuable

information on the spatial and temporal extent of sediment storage in the Colorado River ecosystem.

The average eddy and channel sand thickness through time are illustrated in Figure 7b and c for

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1996-1998, Similar to the high-elevation bar thickness changes described above, we separated the

sample into sites located in upper and lower Marble Canyon. The time-series shows scour of low

elevation areas, the channel and eddies, following high flows greater than power plant capacity (e,g"

the 1996 Controlled Flood), A period of aggradation of channel and eddy sand occurs as sand from

eroding bars is then redistributed back to low elevation environments, Recovery from scour is faster in

eddies because they are more effective traps for sand than the main channel when suspended sediment

concentrations in the mainstem are low (Hazel et ai" 1999; Wiele et al" 1996),

These data suggest that sand eroded from high-elevation was redistributed to lower elevations, or

advected into eddies from upstream sources, resulting in fill of low-elevation areas scoured by the

1996 Controlled Flood, In contrast, the main channel bed at the sites did not begin to aggrade until a

year later (Fig, 7c). 1bis recovery occurred during February and March 1997, when reservoir

drawdown priorities resulted in steady discharges of about 765 m3fs (27,000 fefs) for 21 days, and 680

m3fs (24,000 ft3fs) for 30 days (Fig, 2b), We believe that these high flows were of great enough stage

and duration to erode other areas of temporary bank storage besides eddy bars, such as channel margin

deposits and terraces, Although high elevation bar erosion continued during this two-month period of

sustained high flow, little change in total eddy thickness suggests that eddies were relatively full

because of low elevation aggradation (Fig, 7b),

Subaqueous sand storage changes at the sites were measured in April 1998, about 7 months after

the previous survey (late August, 1997) and 5 months after the 1997 Test Flow, respectively, Because

these data were not collected following the 3 Paria River floods in September 1997, or during the 1997

Test Flow (except in the 3-km reach), we were not able to examine the volume and distributions of

sand supplied by the Paria River at the downstream sites or isolate the effects of the 1997 Test Flow.

For example, whether the 1997 Test Flow produced low elevation scour similar to the response of the

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