6032

• UCREFRP Catalog Number: 6032
• Author: Maddux, H., et al.
• Title: Effects of Varied Flow Regimes on Aquatic Resources of Glen and Grand Canyons
• USFW Year: 1987.
• USFW - Doc Type: Final Report.
• Copyright Material: NO

below a CV of 15%, and the remaining 20% of data points subtend a
CV range from 15% to 100.5%.
The temporal pattern of the mean and CV of daily discharge
at Lee's Ferry was summarized by categorizing these variables
according to "breaks" in their cumulative frequency
distributions. No a priori ecological significance was attached
to boundaries of the classes of either mean or coefficient of
variation in daily flow. The flow categories were assigned
subjectively in order to allow a statistical approach to
measuring the effects of previously undefined "fluctuating
flows". Both measures were divided into low (L), intermediate
(I), and high (H) classes as follows: mean flow 3,487-14,000 cfs
(L); 14,001-31,500 cfs (I); 35,501-51,570 cfs (H); coefficient of
variation 0-3.0% (L); 3.1-15.0% (I); 15.1-100.5% (H).
Three classes in the 3x3 matrix of coefficient of
variation/mean daily flow were poorly represented during the
period of the field study (Table 3.2). Graphical examples of
half-hour interval discharges for representative days in the
remaining six categories are given in Figure 3.2. Of 98 days for
which mean daily flow did not exceed 14,000 cfs, only 3 days had
a CV less than 15%. In contrast, during the 147 days when mean
daily flows exceeded 31,500 cfs, only two days had a CV greater
than 15%.
Daily discharges at Lee's Ferry during April and May 1984
were near 25,000 cfs, at which time they increased markedly to
40,000-45,000 cfs for a two month period. With the exception of
brief periods during August 1984 and October 1984, flows were
again near- 25,000 cfs until March 1985. Excluding the October
1984 period, the CV of daily flows never exceeded 20% and most
days had values of 10% or less (Figure 3.3).
From 12 to 15 August, 1984, discharge at Lee's Ferry
increased to a maximum of 56,600 cfs due to a BOR test of a
reconstructed dam spillway tunnel damaged during the 1983 flood
(Collins 1984). During October 19-22, 1984, discharge from Glen
Canyon Dam was deliberately dropped to near 5,000 cfs in order to
produce a period of low flows for this study. The CV of daily
flows increased to 61% in this interval, but this increase was
attributable to the initial drop and subsequent increase on the
first and final days of the period. CV for the intervening time
did not exceed 1.1%.
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