WSP01181

• File Number: 8220.101.10.D
• Description: Glen Canyon Dam/Lake Powell
• State: AZ
• Basin: Colorado Mainstem
• Date: 10/1/1994
• Title: Experimental High Discharge Release From Glen Canyon Dam Through Grand Canyon NP - Draft Work Plan
• Water Supply Pro - Doc Type: EIS

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research, this risk requires awareness of possible consequences of the experimental process. The
planned high flow experiment may produce unknown effects. These include: (I) the impacts on
the sediment-depleted reach from Glen Canyon Dam to the Paria River; (2)the availability of
sand in narrow reaches of Marble Canyon to which will be delivered to eddies; (3) tbe
capability of the flood to open and regenerating return current channels and backwaters; (4) the
extent ofloss of wetland and riparian vegetation, and associated terrestrial habitat; and (5) the
direct and indirect impacts on endangered species and their habitats, and on riverside cultural
resources. At present, a scientific consensus exists that likely benefits from this event will
outweigh potential losses and that the uncertainties represent acceptable risk.

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SCIENTIFIC BACKGROUND
GeomorpholOGv: the unreaulated river
The Colorado River through Grand Canyon is a gravel-bedded stream that primarily
transports sand and finer sizes as suspended load. The width of the Colorado River is
constrained by bedrock, talus, or debris fan deposits throughout most of its 425-km course
between Glen Canyon Dam and Lake Mead reservoir (Howard and Dolan, 1981). Debris fans,
composed of coarse material supplied from steep ephemeral tributaries, partially block the
channel's course at more than 165 rapids. Channel constrictions result and often form these
rapids, for which the Colorado River is famous.
There is a distinctive assemblage of channel elements related to point source
contributions of coarse material to the river. A backwater of low-velocity flow may extend
several kilometers upstream from the debris fan (Kieffer, 1985). Downstream from the debris
fan, channel cross-section area increases, and large recirculating eddies occur along the channel
banks in these expansions. The downstream termination of these eddies is typically caused by
flow acceleration due to water passage over or around a cobble/gravel bar, narrowing of the
constraining bedrock or talus banks, or where the main channel flow impinges on curving
channel banks. This geomorphic configuration occurs at every tributary mouth, and the size of
each channel element is related to the size and characteristics of each debris fan, the time
sequence of debris flows that replenish the fan, and the time sequence of Colorado River
discharges. Other sources of coarse materials include the channel margin deposits that are not
associated with debris fans and eddies.
Many of the managed resources of Grand Canyon National Park are geomorphically
based and are part of the assemblage described above. Early photographs of Grand Canyon show
that much of the river corridor was comprised of bedrock, talus, debris flow deposits, and bare
sand bars; perennial riparian vegetation primarily existed as linear strips on terraces above about
the 2820 m3/s (100,000 fills) stage, on higher terraces that existed in some locations, and on the
high parts of debris fans. Open sand bars were a distinctive geomorphic feature of the
unregulated river corridor.
Prior to closure of Glen Canyon Dam, the Colorado River varied greatly in discharge
throughout the year. Between I September and I March, mean daily discharge was about 141
mJ/s (5000 fills), but instantaneous peak discharges frequently exceeded 1410 m3/s (50,000 !VIs)
in September and October. The annual peak discharge was caused by snowmelt and occurred. on
average on June 5. The mean annual peak flow for the period 1921 to 1962 was 2180 m3/s

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