WSP05649

• File Number: 8220.101.10.D
• Description: Glen Canyon Dam/Lake Powell
• State: AZ
• Basin: Colorado Mainstem
• Date: 2/1/2000
• Author: Topping et al.
• Title: Colorado River Sediment Transport
• Water Supply Pro - Doc Type: Report/Study

562 TOPPING ET AL: COLORADO RIVER SEDIMENT TRANSPORT, 2
100 RIVER MILE 5.7
--0-- 3-98 r37 mm
<II 10 --e--- g..gs 0.11 mm
<II -ffi-- 5-99 0.32 IMl
:5
<)
W
N
0; 0.1
;;:;
.. 0.01
0.001
0.05 0.1 0.2 0.5 1.0 2.0
GRAIN SIZE (mm)
100 RIVER MILE 6.8
~ 10
<)
~ 0.1
'"
;;:;
.. 0.01
0.001
0.05 0.1 0.2 0.. 1.0 2.0
GRAIN SIZE (nYn)
100 RIVER MILE 7.2
--<>- 3.98 (0.33 nYn)
:z 10 __ ..08 (O.33nYn)
:5 :-!Il- .... (0.26 nYn)
."
~
0; 0.1
;;:;
.. 0.01
0.001
0.05 0.1 0.. 1.0 2.0
100
:z 10
:5
"
W
N
0; 0.1
;;:; --<>- 3-08 (0.48 mm)
.. 0.01 _______ 9-98 (0.30 mm)
~ 5-99 (0.41 mm)
0.001
0.05 0.1 0.2 0.5 1.0 2.0
GRAIN SIZE (nYn)
Figure 14, Measured grain-size distributions of the fine sediment on the bed of the Colorado River in
Marble Canyon at selected locations in March 1998, September 1998 (immediately following the two large
1998 Paria River floods), and May 1999. Sediment introduced by these floods caused a substantial fining of bed
sediment downstream to river mile 6.8 and produced a weaker but detectable secondary mode as far
downstream as river mile 50.1. Bold vertical lines indicate the median grain sizes; values of the median grain
sizes are listed in parentheses.

months later. Preliminary analyses of topographic surveys,
however, suggest that no substantial change in the volume of
sand stored in Marble Canyon occurred between April 1998
and May 1999 (J. E. H32el Jr., personal communication, 1999).
One hypothesis can account for these apparently contradictory
observations of sedtment grain size and sediment volume: If
the total volume of sand in stored in Marble Canyon were
small. then small volumes of newly input sand may cause sub-
stantial changes in grain size without causing detectable
changes in sand storage.

6. Discussion
6.1. Bed-Elevation Changes

To correctly interpret changes in the bed elevation of a river
during a flood, it is important to distinguish changes in bed

elevation driven by local reach-geometric effects from changes
in bed elevation driven by temporal changes in the upstream
sediment supply. The bed at a cross section in a nonuniform
reach typically either aggrades (fills) or degrades (scours) with
a change in water-surface stage. During a flood, bed-elevation
change at a cross section is usually driven by reach geometry
[Colby. 1964; Andrews, 1979; Howard and Dolan, 1981] but can
also be driven by changes in the upstream supply of sediment
[Leopold and Maddock, 1953; Brooks, 1958; Howard and
Down, 1981]. Temporal changes in bed elevation are coupled
to both lemporal changes in the volume of sediment in sus-
pension and spatial changes in the flux of sediment:

o~ I (dV, )
7lt = - c;, at + ~ . Q. ,

(1)