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

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An analysis averaged from single sites may be too small a sample set to accurately represent reach-

scale or canyon-wide patterns of change, Eddies can have high site-to-site variability in sand storage

because low-elevation sand bars are dynamic over short time-scales (Cluer, 1995; Grams and Schmidt,

1999). However, Schmidt et ai, (1999b) showed that individual eddy response during the 1996

Controlled Flood at several sites in this study was similar to the average reach scale behavior

determined from photographic analysis, In addition, observations of channel-bed sediment

distributions using side-scanning sonar indicated that the monitored pools were representative of

observed reach-scale patterns of sand coverage in Marble Canyon in September 1998 (R. Anima,

USGS, personal commun" 1998),

To examine bar deposition as a result of the 1997 Test Flow, the sites were surveyed immediately

following cessation of the test. However, hydrographic mapping was not conducted downstream of the

study reach because the test flow occurred during Grand Canyon National Park's annual non-motor

season (September 15 to December 15), Hydrographic surveys require the use of motorized craft and

were therefore excluded from the post-test flow survey plans. Thus, our analysis of post-I 997 Test

Flow change was limited to high-elevation bar change downstream of the 3-krn reach, In addition to

topographic changes, sediment deposited at each site by the 1997 Test Flow was examined in trenches

and sampled vertically between deposit base and top for grain size changes, The sand was then dry

sieved a 'A phi intervals to determine grain size using the methods of Folk. (1974), Grain-size analyses

of the 1997 samples are also reported by Topping et ai, (2000b),

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THE 1997 PARIA RIVER FLOODS: DEPOSITION AND DOWNSTREAM TRANSPORT

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Deposition in the 3-km Reach Downstream from the Poria River
A significant proportion of the sand delivered to the Colorado River by the August-September
1997, Paria River floods was temporarily stored in the 3-krn reach (Table I), The total volume of sand
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deposited in the study reach during the August 10 flood was 92,800 m (0,16 million Mg), or about 24
to 36% of the estimated sand load of the Paria River (0,56 :!: 0,11 million Mg) for the lO-day period
between field surveys, The total volume of sand deposited in the reach between August 15 and
September 27, was sand from the September 7, IS, and 26 Paria River floods; the volume deposited
was 310,000 m3 (0,53 million Mg). As after the August 10 Paria River flood, the total volume of sand
deposited in the study reach between August 15 and September 27 was about 24 to 36% of the
estimated Paria River sand load (1.85 :!:O,37 million Mg) during this period, Thus, between 64 and
76% of the sand delivered by these tributary floods was quickly transported through the reach and

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