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

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downstream in Marble Canyon. Based on the observations of Topping et ai, (2000b) following Paria

River floods in September 1998, the large fraction of the sand that bypassed this reach was probably

the finer 64-76% (because of their lower settling velocities) of the sand supplied during the August-

September 1997, floods,

Both the rate and volume of sand deposition after the August 10 flood was greatest in Segment 1,

but new sand deposited from the September floods was more evenly distributed amongst the 4

segments (Table I), About 60% of the total deposition resulting from the August 10 flood OCCUlTed in

Segment I, where channel width and eddies are largest. There was little deposition in Segment 4,

Though large amounts of sand were still deposited in Segment I in September, a greater percentage of

the deposition occurred further downstream following the September Paria River floods,

The proportion of new deposition that occurred in the main channel and in eddies varied between

August and September, In August, slightly more than half of the deposition in Segment 1 was in

eddies where as much as 2 m of sediment was deposited, Although eddy bars aggraded in September,

large amounts of sand were deposited on the channel bed, where there was as much as 5 m of

aggradation in Segment I (Fig, 3a) and 6 m of aggradation in Segment 4 (Fig, 3b),

The high bed elevations and flattened channel geometry in the 3-km study reach suggest that the

main channel and eddy environments had filled to near capacity during the two month period of

tributary flooding, Continued sediment input after pool filling, during the same flood or during the

next, was delivered to the next pool downstream or transported completely through the reach, This

process was well documented by Wiele et ai, (1996) following a flood from the Little Colorado River

in January 1993, They found that both the rate and the volume ofsand deposited in the channel was

correlated to pool morphology, Segment I has the widest channel expansion in the 3-km reach with

large eddies along both banks (Eddies I and 2), As a result, Segment I trapped about half of the sand

deposited in the reach, Segment 4 trapped the greatest thickness of sand because this pool has the

deepest scour hole (16 m before the flood inputs at a discharge of 566 mJts), The large increase in

flow depth at scour holes causes vertical expansion of flow and divergence of the boundary shear

stress leading to rapid deposition when sand concentrations in the mainstem are high (Wiele et ai"

1996).

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