9510

• UCREFRP Catalog Number: 9510
• Author: Jones, A. T.
• Title: A Cross Section of Grand Canyon Archeology
• USFW Year: 1986.
• USFW - Doc Type: Excavations at Five Sites Along the Colorado River.
• Copyright Material: NO

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completion of the dam averaged aoout 550,000 tons/day or roughly
ZOO mi 11 i on tons lye ar. However, sedi ment loads vari ed tre1ne ndou sly
depending on 5eason~ discharge fluc~ations. In general, sediment load
increases by the square or cube root of discharge, such that a
three.,fol d increase in discharge will cause sediment load to increase
g to Z7 times. Hence, the river can be expected to do most of its
geomorphic work at flood stage.
According to Leopold (1969), the Colorado River adjusts to
vari ati ons in sediment load and di scharge by adjusting its depth and
vel acity by scour and fill of its bed. Ouring the 9Z, 000 cfs spri ng
flood of 1948, the water el evation rose an average of 3.4 m (11 feetl
while the mean elevation of the streambed fell by 4.9 m (16 feet) due to
scour of channel sediments.
Since the comp1 etion of the G1 en Canyon Oam, fluctuations in
discharge have been on a daily rather than seasonal basis, and the
stream has carri ed consi derab ly 1 ess sediment load. Now di scharge
norma 11 y vari es between about Z, 000 and 28,000 cts, but re ached a hi gh
of 93 ,000 cf sin the summer of 1983 . Lawrence Steven s (1985: pe r so na 1
communication) has observed that an increase in di scharge from 2,000 to
20,000 cts cau se s the ri ver to ri se by about 5 m (15 feet) in the Inner
Gorge, and by as much as 30 m (200 feet) in some places.
Now that Colorado River water is released from the base of Lake
Powell, the Colorado Ri ver is nearly devoi d of sediment load unti'
spring or summer flooding of the Paria and Little Colorado rivers and of
other tributaries to the system. Hence, erosion and removal of beaches
and channel sediments probably is proceeding at a considerablY faster
rate than before the dam was built. Stevens (1982) has observed about
2 to 3 m of bank wi deni ng per year in areas wi th 5andy be aches. Beu s
(1984) has measured the effect of the 1983 92,000 cts 'spill" event on
beach terraces and found that up to 3 m of new sand has been deposited
on eight beaches, while up to 2 m of sand was removed from others. Due
to the underloaded nature of the stream then, the new sand observed on
these beaches must have come mostly from the channel bed itself and/or
frOM other beaches farther upstream.

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