WSP12655

• File Number: 8220.101.10.B
• Description: Colorado River - Water Projects - Glen Canyon Dam-Lake Powell - Glen Canyon TWG
• 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

002356

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Figure 12. Graphs showing cumulative distribution of EDZs in 5 study reaches. A. distribution of all EDZs. B.
distribution for those EDZs larger than 1000 m2.

of eddy deposition. The distribution of EDZ
sizes is log-normal (Fig. 12), and the mean
EDZ area is between 1.3 and 5 times larger
than the median (Table 3). EDZs of moderate
size have the most significant potential storage
area for fine sediment, because the size class
with the greatest cumulative area is that be-
tween 10,000 and 20,000 m2.
The frequency ofEDZs is an indication of
the proportion of fine-sediment deposits that
form in eddies. The frequency ofEDZs is low
in the Lees Ferry and Big Bend Reaches, and a
large proportion of fine sediment occurs as
channel-margin deposits. However, the total
area of eddy bars also depends on the width of
the zone available for fine sediment deposition. .
The largest total area of EDZs per unit river
length occurs in the Tapeats Gorge Reach
where EDZs are the most frequent and the
baseflow channel width is about 70% ofthe
channel width at flood flow. The smallest total
area of EDZs per unit river length is in the
Redwall Gorge Reach where EDZs are rela-
tively frequent but the base flow channel is
about 77% of the channel width of flood flows.

3.3 Sedimentology of Fine-Sediment Alluvial
Deposits

----

Fine sediment of the Colorado River occurs
as terraces and flood plains, and these deposits
can be subdivided by facies and by the minimum
discharge that inundates their surfaces. The
former subdivisions are primarily between eddy
bars and channel-margin deposits. The latter
subdivisions are grouped as (1) pre-dam flood
deposits, (2) post-dam flood deposits, and (3)
post-dam fluctuating-flow deposits. The distri-
bution of these deposits in a specific year was
mapped near Lees Ferry (Hereford et aI., 2000b),
Nankoweap Rapids (Hereford et aI., 1998),
Palisades Creek (Hereford, 1996), and Granite
Park (Hereford et aI., 2000a).
Pre-dam and post-dam fine-sediment
deposits have many similarities. The landforms
of both deposits are primarily those of eddy bars
or channel-margin deposits (Schmidt and Rubin,
1995). Sedimentary structures are primarily
those formed by climbing ripples (McKee, 1938;
Rubin et aI., 1990). Active eddy bars also
include cross-bedding and wave structures;

18 System-wide Changes in the Distribution of Fine Sediment in the Colorado River Corridor ...