WSP00979

• File Number: 8220.101.10.A
• Description: Colorado River-Water Projects-Glen Canyon Dam/Lake Powell-Glen Canyon Adaptive Management-AMWG
• State: AZ
• Basin: Colorado Mainstem
• Water Division: 5
• Date: 3/3/2004
• Author: USGS
• Title: AMWG Meeting Attachments-March 3-4 2004-Computation and Analysis of the Instantaneous Discharge Record for the Colorado River at Lees Ferry-Arizona May 8 1921-Sep 30 2000
• Water Supply Pro - Doc Type: Report/Study

The peak discharges of these iive floods at Lees Feny,
therefore, were smaller or eqoal to the peak discharge of
the 1921 flood that likely produced the "01" deposit,
larger than the peak discharges of all post -1921 floods,
and were in the same discharge range as the eight
paleolloods that left the deposits underlying deposit "0 I."
The fact that the eight prehistoric paleofloods
and the large 1903-20 floods at the Yoma gaging station
were of comparable magnitudes requires that floods of
these magnitodes occurred at least twice in the last 200-
800 years. Thus, the return-period estimates of O'Connor
and others (1994) for the paleolloods are high by a
minimum of a factor of two (herein referred to as
bounding condition one), Extension of the record of
floods at the Lees Feny gaging station from 1921 back to
1903 through use of the record from the Yuma gaging
station, yiclds return periods for these floods that range
from 12 to 60 years (herein referred to as bounding
condition two). Therefore, on the basis of these two
bounding conditions, the return period for the smallest of
the eight paleofloods must range from 12 to 100 years
over the last 2,307 years, and the return period for the
largest of the eight paleofloods must range from 60 to
400 years over the last 2,307 years, Forthermore, the
fact that the 1884 and 1921 floods were larger than the
eight paleofloods suggests that the retorn periods for the
eight paleofloods are closer to those computed on the
basis of bounding condition two (12 to 60 years). The
return-period adjustments that should be applied to the
paleoflood data based on this analysis are indicated in
fig, 36. Finally, although the paleoflood discharges used
by O'Connor and others (1994) were too high and the
return periods computed q.,y 0' Coimor and others were
too long, the upward extension of the flood-frequency
relationship in iig. 36 is quitc similar to that originally
proposed in fig, 5 of O'Connor and others (1994); in both
analyses, a flood with a peak discharge of 300,000 ft3/s
has a return period slightly longer than 1,000 years,
Comparison of the flood-freqoency analyses based
on the partial-duration and annual flood series indicates
that thc analysis based on thc annual flood series
overestimates the return periods for all pre-dam floods
with peak discharges lower than about 60,000 ft3/s
(Jig, 36). The partial-duration flood-frequency analysis
indicates that, prior to the closure of Olen Canyon Dam,
floods with a peak dischargc of about 50,000 ft3/s
occurred every ycar on average, and that floods with a
peak discharge of aboot 125,000 ft3/s occurred every
8 years on average, This analysis also suggests that, under
natural pre-dam hydrology, the retn111 period for the

170,000 Ft3/s 1921 flood was aboot 40 years, the return
pcriod for the 210,000 ft3/s 1884 flood was about 80
years, and that the return period of a 300,000 ft3/s flood
was slightly longer than 1,000 years (fig, 36),

Effect of the Operation of Glen Canyon
Dam on Flood Frequency

To determine the effect of the operation of Olen
Canyon Dam on the frequency of floods on thc Colorado
Rivcr, partial-doration and annual flood-frequency
analyses were also conducted on the post-dam part of the
continuous record of instantaneous discharge (fig, 37A).
The same base discharge was used in the post-dam pattial-
duration flood-frequency analysis as in the pre-dam
analysis (18,500 ft3/s). Doring the post-dam period
between March 14, 1963, and September 30,2000,5,222
floods had peak discharges in excess of 18,500 ft3/s. The
return periods for these 5,222 post-dam floods were
computed on the basis of a 37,6-year period of record,
Because operation of Olen Canyon Dam has effectively
removed the seasonality from the annual hydro graph of
the Colorado River (fig. 30), flood-frequency analysis
based on the annual flood series provides a poor
characterization of the frequency of floods in the post-dam
river (fig, 37A), Compared to the return periods computed
from the pattial-duration Ilood-frequency analysis, the
annoal flood-freqoency analysis greatly overestimates thc
return period of all but the three largest post-dam floods.
The effect of the operation of Olen Canyon Dam
on the frequency of floods on thc Colorado River at Lees
Fcny is shown in fig, 378, Operation of the dam has
increased the retnrn period for floods with pre-dam rcturn
periods in cxcess of about 6-7 months, and has decreased
the return periods for the smallest Hoods in the analysis
(floods with peak discharges of about 18,500 ft3/s) I' TOm
1.8 months to only 2.6 days. The cffcct of dam operations
on the duration of floods with sustained dischargc in
excess of 18,500 ft3/s has also been large (fig, 38),
The longest four periods of sustained discharge above
18,500 ft3/s at Lees Ferry have all occurred since the dam
was closed, In 1984, discharge exceeded 18,500 ft3/s for
214.2 consecutive days; in 199'7, discharge exceeded
18,500 ft3/s for 200,7 consecutive days; in 1983, discharge
exceeded 18,500 ft3/s for 175.0 consecutive days;
and in 1985, discharge exceeded 18,500 ft3/s for 138.4
consecutive days. The longest pre-datn period of sustained
higher discharge occurred in 1929, when discharge
exceeded 18,500 ft3/s for 129,6 consecutive days,

54 Computation and Analvsis of tho Instantaneous-Discharge Record for tho Colorado River at lees Ferry. Arizona-May 8, 1921, through September 30.2000