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

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DATE

F~9ure 28. An example of large dam-induced daily fluctuations in the
discharge of the Colorado River at Lees Ferry.

As a result of the changing operation of Glen
Canyon Dam through time, the discharge characteristics
of the Colorado River at Lees Ferry have changed
substantially during the four decades since closure of the
dam; the median discharge has generally increased, and
the duration of lower discharges has progressively
decreased (fig, 29), Thus, during each decade, as
discharges less than 9,000 ft3/s became less common, the
discharge of the river became progressively more
conducive to the conveyance of sand through or the
erosion of sand from Marble and upper Grand Canyons.
The median discharge for the entire post-dam period of
March 14, 1963, through September 30, 2000, was
12,600 ft3/s. During the initial reservoir-filling period of
March 14, 1963, through December 31, 1970, the
median discharge was 9,490 ft3/s, and the discharge
of the river was greater than 9,000 ft3/s 52,7 percent of
the time. During the 1970s, the median discharge was
11,600 ft3/s, and the discharge of the river was areater
',. " 3 b
than 9,000/t Is 62.2 percent of the time. During the
I 980s, ,the combination of the wet-weather conditions in
the Upper Basin and the full reservoir resulted in the
highest decadal median discharge, 15,900 ft3/s, Discharge
of the river during the I 980s was greaterthan 9,000 ft3/s
75.5 percent of the time, Due to the constraints placed on
dam operations in 1991, the I 990s were the post-dam
decade with the most discharges conducive to the

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SAND
CONVEYANCE
OR
EROSION

10,000

~ 1980s
...J 755%

PRE-DAM Z 1960s t
'- 44.3% ~ 52.7% 1990s

,,\ ::.::.' , :4 197~s ' 082.6%
"""=.::s, - 6./--2,2% I
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.... -
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.,..,....\

SAND
ACCUMULATION

1,000

1000

10 ~ ~ ~ ~ 00 ro 00 00 1(
PERCENTAGE OF TIME EQUALED OR EXCEEDED

EXPLANATION
19605 (March 14, 1963-0ecember 31,1970)
19705 (January 1, 1971-December 31, 1980)
1980s (January 1, 1981-December 31, 1990)
19905 (January 1, 1991-September 30. 2000)
-..- PRE-DAM (May 8, 1921-March 12, 1963)

Figure 29. Flow-duration curves for the Colorado River at Lees Ferry from each
of the four post.dam decades and the entire pre.dam period, The gray shaded
regmn shows the pre-dam discharge range under which sand accumulated in
the reach between the lees Ferry and Grand Canyon gaging stations, in Marble
and upper Grand C~nyons. The ~ercentag~s of time when these discharges
were exceeded durmg each period are mdlcated in italics.

conveyance of sand through or the erosion of sand from
Marble and upper Grand Canyons. Although the median
discharge during the 1990s (13,500 ft3/s) was less than
that during the 1980s, the I 990s had the longest duration
of discharges greater than 9,000 ft3/s (82.6 percent of the
time),

In addition to greatly reducing the duration of
flood flows and progressively eliminating lower flows,
operation of the dam has eliminated the seasonality
from the annual hydrograph of the Colorado River
(fig. 30). Prior to closure of the dam, the lowest discharge
month was January, with a median discharge of 5,140 ft3/s
(fig. 30A) and a minimum discharge of 578 ft3/s (fig.
30B), and the highest discharge month was June, with a
median discharge of 51,200 ft3/s (fig. 30A) and a
maximum discharge of 170,000 ft3/s (fig. 30B). During
the post-dam period of record, the highest-discharge
month was August, with a median discharge of
16,400 ft3/s, and the lowest discharge month was October,
with a median discharge of 10,200 ft3/s (fig, 30A).

48 Computation and Analysis of the Instantaneous-Discharge Record for the Colorado River at Lees Ferry, Arizona-May 8,1921, through September 30, 2000