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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Releases To Mimic "NATURAL CONDITIONS" Have strong
Negative Impacts On Several Resources

It has been suggested that dam operations which mimic
pre-dam flows (i.e. outflow equal to inflow) would lead
to more natural conditions downstream. Such a release
scenario could not be possible under existing
constraints; however, for the purposes of evaluating
the impact of such flows, we have ignored these con-
straints and assumed that the releases could be made in
a pattern similar to that in Figure VII-6. It must be
noted, however, that such releases would still be much
colder and contain much less sediment than pre-dam
river flows.

Humpback chub would probably fare well with a more
"natural" release pattern because flood flows increase
the area of reproductive and rearing habitat at the
mouth of the Little Colorado River. Because flows of
this scenario would be colder than pre-dam flows,
common native fish, which before the dam used the main
channel for larval rearing, would still be dependent on
backwaters for rearing. However, low flows and
associated backwaters would be available in largest
numbers in August, September, and October, a period
very late for larval rearing of common native fish.
Therefore, although flow volume would be similar to the
pre-dam river, the cold water would prevent chub or
common native fish from expanding their spawning beyond
areas currently used.

The trout fishery and fishing would be severely de-
graded. Low flows (3,000-8,000 cfs) would be common
through most of the winter and early spring when
fishing use is heaviest. The periods of very low water
would create relatively undesirable ~ishing conditions
because of reduced access upriver and damage to boats.
These low flows would also substantially reduce, if not
eliminate, trout spawning and rearing.

Frequent large floods combined with the reduced supply
of sediment will greatly reduce streamside terrestrial
habitat and camping beaches. Flood flows would remove
the existing vegetation in a zone between the 31,500
cfs and 100,000 cfs flow levels, and greatly reduce
the area of substrate and beaches on which it grows.
Studies have shown that tributary input of sediments