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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discharges at or greater than the pre-dam mean annual flood, and photographs and observations
of the river at discharges near 2830 mJ/s (100,000 fills) show that many eddies are thin or
non-existent at that discharge. Because eddy deposits are composed of sedimentary structures
indicative of recirculation (Rubin and others, 1990, 1992), eddy bars are formed by flows less
than the magnitudes that drown the controlling constriction. Thus, fine-grain river deposits must
form at discharges less than those that overtop fans or, during the descending limb of a flood
hydrograph. The characteristics of eddy deposits prior to closure of Glen Canyon Dam should
therefore have reflected characteristics of sediment transported during recession from the annual
spring peak and features of lower-magnitude late-summer flood flows.
Eddy bars are subject to scour and fill over various time scales. Interpretation of
sedimentary structures and recovery of scour chains (Rubin and others, 1990, 1992) show that
eddy bars are dynamic features, subject to erosion and deposition during floods. Erosion also
occurs after flood recession but some bars demonstrate greater stability than others and may go
through erosion and deposition cycles within the limits of the river stage. The topographic form
and internal stratigraphy ofbars results from the range of eddy geometries that occur at each site,
which are dependent on channel geometry and discharge (Rubin et al., 1990). Eddy bars
associated with low debris fans that are overtopped frequently by mainstem flooding are likely to
have different scour-and-fill histories than eddy bars in the lee of high -elevation debris fans that
are overtopped less frequently.
The topography of reattachment bars results, at low flow, in a deep channel of stagnant
flow adjacent to the shore and blocked from the main current by the emergent bar. These
stagnant-flow areas increase in nutrient and other concentrations, productivity and biota due
relative to the river due to increased temperature and deposition of silts and clays, hyporheic
percolation through the bars, solar radiation, and greater time of occupancy by water. Backwater
areas are thus potentially important as a rearing habitat for native fishes. However, the
magnitude and dumtion of flows required to initiate opening of return channles are poorly
understood

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GeomorobolO2V: the re21llated river
Closure of Glen Canyon Dam caused a substantial reduction in volume and a change in
the pattern of occurrence of the mean annual flood, and in the volume and pattern of sediment
entering Grand Canyon. The posl-dam 2-yr recurrence flood at Lees Ferry is 795 m3/s (28,100
fillS) for the period 1963 to 1993 but the distribution of post -dam high annual pea1cg, with few
exceptions, has been tied mote to water mRnagement at Glen Canyon Dam than stochastic
events. Water release can be subdivided into "normal" years when less than 1.2 X 1010 m3 of
water are released and annual peak discharges are less than powerplant capacity, and "high"
years when flows 2 or 3 times greater than powerplant capacity occur. Such high-release years
have only occurred in 1965, 1980 and between 1983 and 1986. Ranges of fluctuations associated
with daily production of hydroelectricity is dependent on the annual and monthly volume of
released water. Generally, flows have not fluctuated in high-release years but have fluctuated as
much as 793 ml/s (28,000 fillS) per day in normal-release years (U. S. Bureau of Reclamation,
1994, fig. 11-5),
Randle et al. (1993) computed annual sediment budgets for each year between 1966 and

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