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

the morphology of the area of eddy erosion (Fig. 5), which resembled the shapes of the scars left by the

mass failures described by Cluer (1995) and Andrews et ai, (1999), A semi-circular depression more

than 3 m deep was fanned in the reattachment bar at the downstream end of eddy (Fig, 5b). Failure of

the eddy bar was entirely subaqueous and the high-elevation bar was not affected. Approximately half

of this mass failure was deposited on the slope of the bed between Eddy 3 and the main channel in

Segment 4 (Fig, 3b), On day 2, 12,790 m' of sand were eroded from the main channel in Segment 4,

and an additional 1,800 m' of sand was eroded from Eddy 3, The proportion of these changes that

occurred in Eddy 3 and in the adjacent pool varied over time. Despite deposition in Eddy 3 towards

the end of the test flow, the net change was erosion of 2,570 m' of sand, The net effect of the 2-day

test flow in segments 3 and 4 was erosion (Table 1), The estimated amount of remaining Paria-

supplied sand in the 3-km reach at the end of the 1997 Test Flow was 115,000 m3 (0.20 million Mg),

about 7 to 10% of the 1997, August-September sand inputs,

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Changes in Suspended Sediment Concentrations and Grain Size

Suspended-sediment concentrations in the main-<:hannel at the lower Marble Canyon gage

decreased during the two days of the 1997 Test Flow (Fig, 6a), There was a decrease by a factor of

five in silt and clay concentration (from 0,04% to 0.008% by volume) and a decrease by more than a

factor of three in sand concentration (from 0,07% to 0,02% by volume), In contrast to the sampling 2

months earlier during tributary flooding, sand was the dominant portion of the suspended load during

the test flow, increasing from 62% on the first day to 82% on the second and last day, These results

are remarkably similar to measurements during the 1996 Controlled Flood when the total sediment

concentration decreased the most during the first two days of the flood and sand varied from 73% to

88% of the total suspended sediment (Rubin et ai" 1998; Topping et ai" 1999),

The decrease in suspended-sediment concentration in the main-<:hannel during the 1997 Test Flow

was coincident with an increase in suspended grain-size and bed-material grain-size, The median grain

size of the suspended sediment increased from 0,09 to 0,105 mID and the bed particles increased from

0,27 to 0,3 nun, mostly during the first day (Fig, 6b). This same pattern was observed during the 1996

Controlled Flood and is thought to result from depletion of fine-grained sediment from the channel

bed, either by deposition at higher elevations along the channel margin or transport through the canyon

(Rubin et ai" 1998; Topping et ai" 1999), The main difference between the suspended-sediment

grain-size evolution during the two flood experiments is that at the start of the 1997 Test Flow both the

suspended sediment and the bed material at the lower Marble Canyon gage was finer, At the end of

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