WSP05604

• File Number: 8220.101.10.D
• Description: Glen Canyon Dam/Lake Powell
• State: AZ
• Basin: Colorado Mainstem
• Date: 1/1/1999
• Author: Topping et al.
• Title: Linkage Between Grain-Size Evolution and Sediment Depletion During Colorado River Floods
• Water Supply Pro - Doc Type: Report/Study

76 GRAIN-SIZE EVOLUTION DURING FLOODS

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60
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A

,y.. Day-' (n=5)
o Day 1 (n = 5)
. Day.2 (n = 4)

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Day 3 (n = 5)
Day 5 (n = 3)
Day 6 (n = 4)

Day 7 (n = 4)
_- - Day.! (n = 3)

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.

1923-1996 Paria River sand input (D,o = 0,13 mm)
4-12-56 (D", = 0,2 mm)
5-31-56 (D", = 0,4 mm)
6-25-83 (D,o = 0,7 mm)

0,1

Grain Size (mm)

Figure 2. (A) Daily mean grain-size distributions of the bed sediment at the Grand Canyon gage during the 1996
controlled flood. During the rising limb oflhe flood. the median size of the bed sedimenl fined; during the 7 days of high
discharge, the median size of (he bed sediment coarsened. mainly during (he first 2 days of high discharge. The number
of samples in each spatially averaged measurement is indicated by n in the legend; the thick venicallines indicate the
median grain sizes for each day. (8) Grain-size distributions of: (he calculaled 1923-1996 Paria River input of sand 10 the
Colorado River (after Toppitrg. 1997]; the bed sediment at (he Grand Canyon gage on 4-12-56. during the rising limb of
the 1956 snowmelt flood [US, Ceological Survey. 1961]: the bed sediment at the Grand Canyon gage on 5-31-56, at
near-peak disch3'lle of the 1956 snowmelt flood [US, Ceological Survey. 1961]; and, the bed sediment at the Grand
Canyon gage on 6-25-83, at near-peak discharge of the 1983 flood [Carrett and o,hers. 1993], The thick vertical lines
indicate the median grain sizes in each distribution. The grain-size distributions of the 1956 and 1983 samples were
calculated at If2~$ increments by weighted cubic-spline interpolation so that they could be more easily compared to the
grain-size distribUlions in (A).

peak discharge of 2720 mils during the 1983 flood, Prior to
coarsening during the 1996 controlled flood. the bed under
the measurement cable way at the Grand Canyon gage first
fined as it aggraded with the initial increase in waler-
surface stage (from day -I to day 1 in Figure 3), The
sources of this fine sand were presumably the upstream
channel bed. bars, and banks that eroded during the rising
limb of the flood. Following this initial aggradation and
fining, the oed under the measurement cahleway at the

Grand Canyon gage coarsened (Figures 2 and 3) prior to
any significant scour of the bed,
Bed coarsening at the Grand Canyon gage during the
1996 controlled flood was accompanied by both a coars-
ening of the suspended sand and a decrease in suspended-
sand concentration (Figures 3 and 4, Tables 1 and 2),
Analyses of pre-dam sediment data at the Grand Canyon
gage indicate that. as during the 1996 controlled flood,
decreasing suspended-sand concentrations during the