7387

• UCREFRP Catalog Number: 7387
• Author: U.S. Bureau of Reclamation.
• Title: Glen Canyon Environmental Studies Executive Summaries of Technical Reports.
• USFW Year: 1988.
• USFW - Doc Type: \
• Copyright Material: NO

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been as large as 400,000 cfs) no longer occur. There-
fore, for discussion of the rapids, it is convenient to
think of the dam discharges during three periods: (a)
pre-dam, (b) prior to filling of Lake Powell to opera-
tional level (1962-1983), and (c) after filling of the
lake (1983 to future).

The velocity and streamline data obtained in this study
suggest that boulders on the order of 3 ft diameter can
be moved from the main channel at even low discharges
(e. g., from considerations of the Hjulstrom criterion
for boulder transport and unit stream power of the
river, the velocities of 20-25 ftjs measured at 5,000
cfs at House Rock Rapid [Figure 2] are adequate to move
3 ft boulders out of the narrowest part of the channel
at that location). The competence' of the river, even
at relatively low discharges, approaches that of some
of the largest floods in the world inferred from paleo-
hydraulic reconstruction (e.g., Baker 1984). For the
Colorado River, then, we have the opportunity both to
use paleohydraulic knowledge to constrain our models
where the scale of time and space are both too large
for laboratory study, and to contribute to the under-
standing of rare geologic events in the past by moni-
toring and understanding of this large and powerful
river.

At Crystal Rapid, the powerplant discharges from 1966
to 1983 cut only a relatively narrow channel through
the debris fan. In its narrowest part, the channel was
only about one-quarter the width of the unconstricted
channel upstream of the rapid. This single example
suggests that, if discharges were held to powerplant
releases, the rapids would become severely constricted
with time, and that the river channel would be
approximately twice as constricted as in pre-dam time.

Discharges larger than powerplant releases will widen
the river channel back toward the pre-dam geometry of
the channel. The 1983 high water of 92,000 cfs
enlarged the constriction at crystal Rapid from a value
of approximately 0.25 to approximately 0.40. Extra-
polation of the calculations for Crystal Rapid to
higher discharges suggests that floods on the order of
400,000 cfs have contoured the channel of the Colorado
River to its present shape at the older debris fans
(Kieffer 1985). Spring releases of 50,000-60,000 cfs
and 1983-level releases of 100,000 cfs will widen
severely constricted places (e. g., a constriction of
0.25 might be enlarged to 0.40 by a release of 92,000
cfs). However, unless releases approach 300,000-
400,000 cfs the rapids will eventually become more