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

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PERCENTAGE OF TIME EOUALED OR EXCEEDED

Figure 3. Graph showing flow duration curves of the
Colorado River at the Lees Ferry gage for the pre-dam
and post-dam periods (Topping et aI., 2003).

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. JULY 21-MAY 31 MEASUREMENTS
o JUNE 1 -JULY 20MEASUREMENTS

TOTAL SAND (0.0625-2.0 mm)

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LEES FERRY GAGE

100 1000
DISCHARGE, IN CUBIC METERS PER SECOND

TOTAL SAND (0.0625 - 2.0 mm)

GRAND CANYON GAGE

100 1000
DI!'>CHAR(,;E IN CUBIC METER!'> PER !'>ECClND

Figure 4. Graphs showing pre-dam sand concentrations
as a function of water discharge for the Lees Ferry and
Grand Canyon gages. Cross-hatched region overlaying
the Grand Canyon gage data indicates the region in
concentration-discharge space occupied by the Lees
Ferry data. (Topping et aI., 2000b, Fig. 4B).

concentration at any given discharge was 2
orders of magnitude at the Grand Canyon
gage, whereas this variation was about half an
order of magnitude at Lees Ferry.
The relative concentration of fine sedi-
ment at the two gages differed between the
low-flow and flood-flow seasons. When flows
were less than about 250 m3fs, the concentra-
tion of sand in suspension was greater at the
Lees Ferry gage than at the Grand Canyon
gage (Fig. 4). At flows less than about 150 m3f
s, the concentration of suspended sand at Lees
Ferry was about 2 orders of magnitude more
100 than at the Grand Canyon gage. When flows
exceeded about 500 m3/s during the rising limb
of the annual spring flood, the concentration of
suspended sand at the Grand Canyon gage
exceeded that at the Lees Ferry gage. The
concentration of sand in suspension at the
Grand Canyon gage subsequently decreased to
approximately equal the concentration of sand
in suspension at the Lees Ferry gage.
These seasonal differences in concentra-
tion imply that there was a 9-mth period
between July and the following March when
sand accumulated, because more sand was
delivered into Marble and upper Grand Can-
yons than was exported downstream (Topping
et aI., 2000b). This was the period when
discharge was mostly less than 250 m3/s.
During the spring snowmelt flood between
April and June, the amount of sand exported
past the Grand Canyon gage was approxi-
mately equal to the amount transported past the
Lees Ferry gage plus the amount of sand that
had accumulated in the reach since the previ-
ous July. Decrease in the concentration of
sand in transport past the Grand Canyon gage
during the spring flood resulted from depletion
of the supply that had accumulated during the
preceding low-flow season. Thus, fine-sedi-
ment deposits between the gages were eroded
and exported downstream during the annual
spring snowmelt flood. Presumably, the total
amount of sand on the bed and along the banks
in Marble and upper Grand Canyons was least
immediately upon recession of the snowmelt

2.0 Water and Fine-Sediment Fluxes 5