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<br />- Lees Ferry
<br />- - Redwall Gorge
<br />. . - - - PI. Hansbrough
<br />- - . - T apeats Gorge
<br />__.m Big Bend
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<br />5 10 2030 50 70 BO 9095 99 99.999.99
<br />
<br />PERCENT SMALLER THAN INDICATED SIZE
<br />
<br />A.
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<br />- - Redwall Gorge
<br />- . , , . Pt. Hansbrough
<br />. -. - Tapeets Gorge
<br />.-- --- Big Bend
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<br />
<br />Figure 12. Graphs showing cumulative distribution of EDZs in 5 study reaches. A. distribution of all EDZs. B.
<br />distribution for those EDZs larger than 1000 m2.
<br />
<br />of eddy deposition. The distribution of EDZ
<br />sizes is log-normal (Fig. 12), and the mean
<br />EDZ area is between 1.3 and 5 times larger
<br />than the median (Table 3). EDZs of moderate
<br />size have the most significant potential storage
<br />area for fine sediment, because the size class
<br />with the greatest cumulative area is that be-
<br />tween 10,000 and 20,000 m2.
<br />The frequency of EDZs is an indication of
<br />the proportion of fine-sediment deposits that
<br />form in eddies. The frequency ofEDZs is low
<br />in the Lees Ferry and Big Bend Reaches, and a
<br />large proportion of fine sediment occurs as
<br />channel-margin deposits. However, the total
<br />area of eddy bars also depends on the width of
<br />the zone available for fine sediment deposition. .
<br />The largest total area of EDZs per unit river
<br />length occurs in the Tapeats Gorge Reach
<br />where EDZs are the most frequent and the
<br />baseflow channel width is about 70% of the
<br />channel width at flood flow. The smallest total
<br />area ofEDZs per unit river length is in the
<br />Redwall Gorge Reach where EDZs are rela-
<br />tively frequent but the base flow channel is
<br />about 77% of the channel width of flood flows.
<br />
<br />3.3 Sedimentology of Fine-Sediment Alluvial
<br />Deposits
<br />
<br />.
<br />
<br />Fine sediment of the Colorado River occurs
<br />as terraces and flood plains, and these deposits
<br />can be subdivided by facies and by the minimum
<br />discharge that inundates their surfaces. The
<br />former subdivisions are primarily between eddy
<br />bars and channel-margin deposits. The latter
<br />subdivisions are grouped as (1) pre-dam flood
<br />deposits, (2) post-dam flood deposits, and (3)
<br />post-dam fluctuating-flow deposits. The distri-
<br />bution ofthese deposits in a specific year was
<br />mapped near Lees Ferry (Hereford et aI., 2000b),
<br />Nankoweap Rapids (Hereford et aI., 1998),
<br />Palisades Creek (Hereford, 1996), and Granite
<br />Park (Hereford et aI., 2000a).
<br />Pre-dam and post-dam fine-sediment
<br />deposits have many similarities. The landforms
<br />of both deposits are primarily those of eddy bars
<br />or channel-margin deposits (Schmidt and Rubin,
<br />1995). Sedimentary structures are primarily
<br />those formed by climbing ripples (McKee, 1938;
<br />Rubin et aI., 1990). Active eddy bars also
<br />include cross-bedding and wave structures;
<br />
<br />.
<br />
<br />~8 System-wide Changes in the Distribution of Fine Sediment in the Colorado River Corridor...
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