WSP12639

• File Number: 8220.101.10.A
• Description: Colorado River - Water Projects - Glen Canyon Dam-Lake Powell - Glen Canyon AMWG
• State: CO
• Basin: Colorado Mainstem
• Water Division: 5
• Date: 10/1/2004
• Author: Schmidt - Topping - Grams - Hazel
• Title: System-Wide Changes in the Distribution of Fine Sediment in the Colorado River Corridor Between Glen Canyon Dam and Bright Angel Creek - Arizona - Final Report - 10-01-04
• Water Supply Pro - Doc Type: Report/Study

. Table 4. Estimates of the potential fine-sediment storage volume in 9 EDZs.
EDZ name EDZ Area Area of Void volume Percent
(NAU designation) area, In surveyed comparison, between the stage overlap
square by NAU, in square of 100 m3/s and the between
meters in square meters minimum elevations EDZ and
meters surveyed by NAU, area
in cubic meters surveyed
by NAU
Cathedral 11,658 8392 7124 25,122 72
(RM3)
Fence Fault 11,479 9448 4954 8949 82
(RM 30)
South Canyon 10,837 9536 4316 11,877 88
(RM 32)
Anasazi Bridge 25,348 11,318 4545 12,412 45
(RM43)
Eminence Break 80,259 30,377 12,884 34,776 38
(RM 45)
Saddle Canyon 44,977 29,935 21,831 92,797 67
(RM47)
Crash Canyon 20,103 17,816 14,878 92,787 89
(RM 62)
Carbon 20,253 18,123 10,971 24,451 89
(RM 65)
Tanner 11,476 9422 4269 11,822 82
(RM 68)

001584

Thickness
of void
v1:llume, in
meters

3.53

1.81

2.75
2.73

2.70

4.25
6.24
2.23
2.77

.

Eddies have an enormous capacity to
store fine sediment. They have the potential to
store as much fine sediment as Topping et al.
(2000b) estimated accumulated seasonally
prior to completion of Glen Canyon Dam. We
determined the composite lowest elevation
surface surveyed by NAU in the 1990s in 9
EDZs by integrating all surveys and ,determin-
ing the lowest measured value at each grid
node. We subtracted this topographic surface
from a flat surface whose elevation is the stage
at 100 m3/s. This flat surface conforms to the
lowest stage used in the calculation of EDZs.
The difference between these two surfaces is a
conservative estimate of the fine sediment
storage potential in the eddy. The volume
between these two surfaces, divided by the
area of comparison, is between 1.8 and 6.2 m
(Table 4). The mean thickness is 3.2 m.
Application of this thickness to the total
estimated area of EDZs yields estimates of
8.32 x 106 and 4.2 x 106 m3 for the potential
storage volume of eddies in Marble and upper
Grand Canyons, respectively. The potential

.

storage volume is 13.1 x 106 and 6.5 x 106
metric tons for Marble and upper Grand
Canyons, respectively. These values approxi-
mate the upper range of seasonal fine-sediment
accumulation estimated by Topping et al.
(2000b ).
There is substantial longitudinal variation
in the number and size ofEDZs. The fre-
quency of EDZs largeF than 1000 m2 varies
twofold among the reaches (Table 3). There are
2.2 EDZs larger than 1000 m2fkm in the Lees
Ferry Reach, and there are 5.6 EDZs per
kilometer in the Tapeats Gorge Rea(;h. The
largest EDZ in these reaches is 67,000 m2 and
occurs immediately downstream from Paria
Riffle, at RM l.2R (Fig. 7). The second largest
EDZ is located between Comanche and Tanner
Creeks in the Big Bend Reach at RM68.2L;.
this EDZ is 52,800 m2 in area. Other large
EDZs are Saddle Canyon (RM47.6R; 45,000
m2) and Triple Alcoves (RM47.1R; 43,300 m2).
Although there are some very large EDZs in
the study area, these are so few that they do not
account for a large proportion of the total area

3.0 The Valley of the Colorado River 17