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

Finnl Report

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Canyon gage (Topping et al" 2000b), This change occurred because the November 1997, sand-
transport rates at the Lower Marble Canyon gage were twice that observed during the 1996 Controlled
Flood. Thus, the large sand inputs from the Paria River in 1997 resulted in a doubling of the sand
export rate from Marble Canyon during the 1997 Test Flow, The estimated sand transport was 70,000
m3 (0,19 :t 0,04 million Mg), or about 9% of the 1997 Paria River sand inputs, In addition, net sand
deposition above the 566 m3fs (20,000 fefs) stage elevation was significantly less than that achieved
throughout Marble Canyon by the 1996 Controlled Flood (discussed in detail below),

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High-Elevation Sand Bar Changes

Sand bars downstream from Glen Canyon Dam attain elevations and volumes directly related to

flow magnitude and adjust vertically according to changes in dam operation, To examine temporal

changes in high-elevation sand bar thickness we integrated the results of this study with measured

changes since 1996, for Marble Canyon (Fig. 7a), Because the gradient and channel width of the

Colorado River changes greatly near river mile 38 (Schmidt and Graf, 1990; Melis, 1997), we divided

the sample sites into two populations: those in upper Marble Canyon (6 bars) and those in lower

Marble Canyon (8 bars),

The time series demonstrate that sand was successfully redistributed to high-elevation by the 1996

Controlled Flood (Fig, 7a), The average thickness increase was 0,5 m in upper Marble Canyon and

0.7 m in lower Marble Canyon, During the interval between the 1996 Controlled Flood and the 1997

Test Flow, readjustment of the newly aggraded bars to lower, sustained high flows led to rapid but

declining rates of erosion (also see Kaplinski et al" 1998). As a result, nearly all of the flood-related

deposition in upper Marble Canyon was eroded, whereas in lower Marble Canyon, the magnitude of

erosion was substantially less and one-third of the sites measured eroded,

The 1997 Test Flow did not result in aggradation great enough to compensate for the erosion that

had occurred between April 1996 and November 1997 (Fig. 7a). Net high-elevation bar thickness did

not increase at the sites because deposition of sand on the inundated part of the bar was offset by

erosion of high-elevation parts of the preexisting deposits (Fig, 8), In general, as much as I m of

deposition was located at the downstream parts of eddies where recirculating flow reattaches to the

bank, Erosion occurred as the result of cutbanks that retreated horizontally as much as 5 m, The base

of the cutbanks developed at the stage elevation reached by the 878 m3/s flow. The high-elevation

erosional trend evident in the time series in 1996 and 1997, suggests that potential depositional area

was open, especially in upper Marble Canyon (Fig, 7a). The lack of net deposition, despite high sand

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