WSP00054

• File Number: 8220.101.17
• Description: Glen Canyon Dam/Lake Powell
• State: AZ
• Basin: Colorado Mainstem
• Date: 3/7/2001
• Title: Sediment Delivery by Ungaged Tributaries of the Colorado River in Grand Canyon - Summary
• Water Supply Pro - Doc Type: Report/Study

grained sediment into the river (Webb and
others, 1999).
The resuhs of this model indicale that
debris flows deliver 0.14--0.30'106 Mglyr
of sediment to the main channel. Reach B
(Marble Canyon) contributes the greatest
amount of debris-flow sediment, which is
consistent with both the empirical
observations on where debris flows have
occurred in the last century as well as the
mapped disuibution of probabilities in
Grand Canyon (Griffiths and others.
1996). Depending upon the assumptions
of the debris-flow sediment-yield model,
sediment yield by debris flow ranges from
4 to 23 percent of lotal sediment yield.

Particle-Size Distributions

The size of the sand fraction is of
particular interest for the management and
restoration of sand bars in Grand Canyon.
Measurements of particle-size distri-
butions Slored in stream terraces in various
tributaries, as well as suspended sediment
samples from Bright Angel Creek and
other small tributaries, provide sand
contents ranging from I to 99 percent with
no discernible pattern. These data were
collected from a large discharge range and
thus highly variable sand contents would
be expected. An average sand content of
50 percent of total stream/low sediment
yield was used in this study, which
compares favorably with average sand
content weighted by discharge for the
Little Colorado and Paria Rivers (30 and
50 percent, respectively). Sand contents of
IS, 50, and 7S percent are reported. Sand
delivery by streamflow from the Glen and
Marble Canyon reaches averages about
0.032'106 and 0.305'106 Mglyr,
respectively (0.34'106 total). with a
combined total of the two reaches ranging
from 0.10-0.51'106 Mglyr, depending on
the assumed sand content. Sand
contributed by tributaries in Glen Canyon
is notably coarser (D50=0.24 mm) than
sand in other reaches (D50=0. II - 0.20
mm), including the Marble Canyon reach
(D50=0.20 mm) (fig. 4).
The particle-size distributions of 41
fresh, unaltered deposits of debris flows
that occurred between 1965 and 1999
were determined. Pebbles are the most
abundant particles at 41 percent by weight.
and boulders typically account for about
14 percent. The sand content of debris

1 00 10' 1 02

Drainage Area (km2)

Figure 3. Comparison of methods for calculating streamflow sediment yield. A. Gaging
stetion and reservoir data from the Colorado Plateau region (,2 = 0.86). B. Comparison
of regional deta regression equation, Renard (1972) equation, end estimetes for 768
ungeged tributaries by the flood.frequency rating-curve method.

106
10.
104
103
102
~
>-
-
en 10'
~
."
Qj 100
>= 106
'E
Q)
E 10.
'C
Q)
C/) 104
103
102
10'

100
10-2

A

. .

.

.
'"

Small Reservoirs
Gaging Stations

,

B

Regional data regression
- - - - - Renard (1972) equation
o Flood-frequency method

10"

1 03

1 04

100
~
Q) 80 'x
c:
IT: ~
'E
Q) 00
~
Q) Reach A (n = 6)
0.. - Reach B (n = 71) )(
Q) 40
>
:; - - - - - Reach C (n = 6)
:; . Reach D (n = 3)
E 20
::l - Reach F (n = 3)
(.) - - )(.
0
2 1 0.5 0.25 0.125 0.063
Particle Size (mm)

Figure 4. Particle-size distribution of sand delivered by streamflow from ungaged
tributaries. Sand Input by streamflow is increasingly finer downstream from the dam.

flows averages about 18.2 percent and
ranges from 2 to 47 percent. With debris-
fan reworking limited by the operation of
Glen Canyon Dam, debris flows transport
0.006-0.013'106 Mglyr of sand to the
regulated Colorado River, while 0.023-

0.048'106 Mglyr is stored in unreworked
pans of debris fans. Depending on the
volume model used and the amount of
debris-fan reworking, the total sand yield
of debris flows in all reaches ranges from
0.006-0.054'106 Mglyr. Although debris