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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/' WATER YEAR 1934
.----WATER YEAR 1936

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1920 1930 1940 1950 1960
WATER YEAR

1970 1980 1990

Figure 17. Number of days when the daily mean discharge record at Lees
Ferry was modified on the basis of the dIScharge record at the Grand
Canyon gaging station. This figure includes days when discharge was
modified because of ice conditions and other reasons, such as during 1934
and 1936 when the computed discharge on many days in the spring and
summer was reduced at Lees Ferry.

Methods Used in this Study to
Compute the Continuous Record of
Instantaneous Discharge for
Water Years 1921-86

Following review of the methods used by the
USGS to compute the daily mean discharge record,
the first step in computing the continuous record of
instantaneous discbarge was to constmct a continnous
record of instantaneous stage from May 8, 1921, through
September 30, 1986, relative to the datum of thc Dugway
and Lees Ferry Gages established on September 21,1921.
For the period prior to installation of the continuous stage
recorder on January 19,1923, this step involved first
entering the stages measured at the three principal staff
gages in the reach (i.e" the LaRue Gage. Number I Gage,
and Dugway Gage) into computer tiles from the Oliginal
USGS field notebooks (see Appendix B for additional
background information on these staff gages). Each staff
gage had a different period of record and datum; the
LaRue Gage was installed on May 8,1921, and was
destroyeddming the June 1921 flood; the Number 1 Gage
was installed at the site of the LaRue Gage on June 24,

1921, but at a different datum; the Dugway Gage was
installed on August 5,1921, near the future location
and at thc same datum as the Lees Ferry Gage. Thus,
for the peliod prior to the beginning of record at the
Dugway Gage on August 14, 1921, the stages measured
on the LaRue and Number I Gages were converted
into equivalent stages on the Dugway Gage. This was
accomplished by tirst convcl1ing the stages measured on
the LaRue Gage into stages on the Number 1 Gagc
by a process described in Appendix D. Then, the stages
measured on cither the LaRue or Number I Gages plioI'
to August 14, 1921, were converted into equivalent stages
on the Dugway Gage on the basis of the relationship
betwecn the Number 1 and Dugway Gages developed
by the USGS on January 25, 1922.
For the period after installation of the Lees Ferry
Gage on January 19, 1923, with its continuous strip-chart
stage recorder, construction of the continuous record
of instantaneous stage involved digitizing all of the
stage-recorder graphs between January 19, 1923, and
September 30, 1986, such that all major breaks in
slope of the continuous stage record were captured,
These digitized stage records werc then corrected for time
and pen errors. During periods of ice effcct, the stage-
recorder graphs were digitized so that most, if not all, of
the ice effect was removed from the stage record. For
example, the period in fig. 16C was digitized by following
the dashed pencil line that was originally drawn on the
record by the USGS to COlTect for the ice-dlivcn lises in
stage. During the two peliods when large floods on the
Pari a River caused backwatered flow conditions in the
Colorado River at the Lees Feny Gage (on October 5,
1925, and on August 2, 1929; Appendix B), the resulting
increased stages were removed from the digitized stage
record by the same method. To augment and check the
stage records digitized from the stage-recorder graphs. the
digital stage-recorder records from Febmary 2, 1967.
through September 30, 1986, were also entered into
computer tiles.
The second step in this process was to convert the
continuous record of instantaneous stage into a continuous
rccord of instantaneous discharge by using the stage-
discharge rating curves and shift curvcs Oliginally used by
the USGS to compute the published daily mean
discharges. To accomplish this, all of the rating curves and
shift curves were entered into computer files and then
were used to compute instantaneous discharge for the
period from May 8, 1921, through September 30, 1986.
Finally, the discharges at Lees Ferry were reduced to

Computation of the Continuous Record of Instantaneous Discharge at Lees Ferry faT Water Years 1921-2000 37

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