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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than than 9,000 cfs are not released nor are they
likely to be in the future. Such low flows occur only
as part of a pattern of fluctuations. Under current
operations, 'minimum releases are delivered each year
in a pattern of fluctuating releases much like those
shown for 1982 (Figure VI-1b).

Definition of Current Operations. To simplify and
quantify the pattern of current releases, we created a
representat~ve sequence of years with three low-water
years for everyone high-water year (Figures VI-1a and
VI-1b). The actual releases of 1982 (8.25 maf) and
1986 (16.6 maf) were selected to represent the
low-water year and high-water year, respectively. Each
low-water year has year-round fluctuating releases with
no spring 'flood, whereas each high-water year has many
months of high steady releases (20,000 to 31,500 cfs) ,
few months of fluctuating releases, and a spring flood
release of 40,000 to 50,000 cfs for four to six weeks.

Basis for evaluation of current operations. The effect
of flood releases was assessed by comparing the impacts
of the current operations sequence described above
against the impacts of the same sequence with the
spring flood releases removed. In order to keep the
annual release volume constant, the flood volume of
about 1.0 maf was spread evenly throughout the year,
increasing releases slightly (1,000-2,000 cfs).
similarly, the baseline for evaluation of fluctuations
was the current operations sequence with all daily
fluctuations converted to steady releases with the same
daily volume.

The flow sensitive aspect of each critical resource is
given in Table VI-1. This table also presents the part
of the year (sensitive period) in which the aspect used
for evaluation is most affected by flows. For some
resources, the sensitive period encompasses the entire
year. The measure of the flow sensitive aspect is
given in the table as well. For each resource, an in-
crease in the measure corresponds to a positive impact.
For example, the flow sensitive aspect for humpback
chub is the area available for spawning and rearing in
the mouth of the Little Colorado River; the measure is
population size, and an increase in population size is
a positive impact.