7423

• UCREFRP Catalog Number: 7423
• Author: U.S. Bureau of Reclamation.
• Title: Glen Canyon Environmental Studies Draft Report, September 1987.
• USFW Year: 1987.
• USFW - Doc Type: Washington, D.C.
• Copyright Material: NO

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because the supply of sediment is severely reduced and
is highly erratic. Floods may also irreversibly impact
the vegetation by leaching nutrients from the soil
(Appendix B, Section V) Nutrient-poor soil could
limit productivity, change the species composition of
riparian vegetation, and lead to loss of wildlife
habitat and a decrease in the diversity and abundance
of plants and animals. .

Although some potential benefits of infrequent flood
releases have been hypothesized, it appears. that the
detrimental effects of even rare floods (1 in 20 years)
outweigh potential benefits. We have had only a short
time to monitor the response of the system to floods,
and therefore have very little idea of how the system
responds to a given frequency of floods. However,
based on evidence of irreversible damage from the 1983
flood, which occurred after 20 years without floods, we
conclude that floods occurring more frequently than
once in 20 years will result in loss of critical re-
sources without substantial benefits. These potential
benefits are discussed below.

(1) As vegetation ages and becomes more
homogeneous, the diversity of animals that
depend on vegetation is often reduced.
Infrequent flooding may open areas for
colonization by younger individuals of the
same plant species or different species, thus
increasing vegetation habitat diversity, and
in turn increasing ~nimal diversity. The
frequency of flooding that would enhance
diversity is not known at the present time.
We know from this study that diversity within
the animal community along the river was
increasing from 1963 to 1982, a twenty-year
period of operations with almost no flood
releases, we do not know how long plant and
,animal diversity would have continued to
increase or whether the flood in 1983 will
increase or decrease diversity in the long
term.

(2) If sand is in sufficient supply in and
along the river, floods can move sand from
low elevations to high elevations where it is
more useful for campsites. Redistribution of
sand during floods cleans it of refuse and
scours away any encroaching vegetation which
may make camping more difficult. However,
overall beach area will be lost during any

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