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<br />downstream from Lees Ferry) on June 28, the correct date
<br />for the peak of the 1884 flood at Lees Ferry is probably
<br />June 18, Estimation of the peak magnitudes of the 1884
<br />and 1921 floods depends on the accurate determination of
<br />maximum flood stages and appropriate extrapolations of
<br />the stage-discharge rating curves at the various staff gages
<br />in the Lees Ferry reach. Because of the importance of the
<br />1884 and 1921 floods, all available information about the
<br />magnitudes of the peak stage and the methods by which
<br />discharge was estimatecl from the stage data for each of
<br />these floods was assemhled and reviewed,
<br />
<br />Estimate of the Peak Discharge of the
<br />1921 Flood at Lees Ferry
<br />
<br />Because estimation of the peak discharge of the
<br />1884 1100d depends on estimation ofthe peak discharge
<br />of the lower June ]921 flood, estimation of the peak
<br />discharge of the 1921 flood is described first. The peak
<br />discharge of the 192J nood at Lees Ferry was unknown
<br />and had to be estimated, but the peak stage was precisely
<br />measured at the Number 1 Gage, the Dugway Gage,
<br />and at two different locations at the Lonely Dell Ranch.
<br />Analysis of photographs taken of the Number 4 and Cable
<br />Gages in 1921-24 (Appendix B) allow the peak stage of
<br />the] 921 flood to be determined at these two additioual
<br />sites, but with less precision (fig, 4A).
<br />The original USGS estimate of the peak
<br />discharge of the June 19211100d was 174,000 ft3/s; this
<br />value was based on extrapolation of the first available
<br />stage-discharge rating turve for the Number J Gage
<br />(unpublished USGS 1921 annual technical file for the
<br />Lees Ferry gaging station), This rating curve was based on
<br />120 discharge measurements made between August 5,
<br />1921, and June 15, 1922, The original estimate of
<br />174,000 ft3/s was not published. The first published
<br />estimate of the peak discharge of the 1921 flood at Lees
<br />Ferry was "about 190,000 ft3/s " (Grover and others,
<br />1923); this value was based on comparison of the
<br />estimated daily mean discharges during the] 921 flood
<br />at Lees Ferry with those measured downstream at the
<br />'Ibpock and Yuma gaging stations, This value of "ahout
<br />190,000 ft3/s " was the official USGS estimate oUhe peak
<br />discharge of the June 1921 flood until 1939, when the
<br />USGS revised the estimate to 220,000 ft3/s (Grover and
<br />others, 1939), on the basis of the 1938 study of lS.
<br />
<br />.
<br />
<br />Gatewood and R,S. Hunter. The 1939 revision remains
<br />the emrently accepted estimate of the peak discharge of
<br />this flood.
<br />
<br />The Key Assumption of J,S. Gatewood and
<br />R.S, Hunter
<br />
<br />The 1939 revision of the estimate of the peak
<br />discharge of the 1921 flood was based on a key
<br />assumption now known to be false, Because the reversal
<br />in the curvature ofthe stage-discharge rating emve at
<br />the Lees Ferry Gage made extrapolation of this curve
<br />difficult, Gatewood and Hunter decided to estimate the
<br />peak discharge of the 192] flood at the Grand Canyon
<br />gaging station. They chose the Grand Canyon gaging
<br />station over Lees Ferry as the place to perform this
<br />extrapolation because (I) the stage-discharge rating curve
<br />was more stable at the Grand Canyon gaging station than
<br />at the Lees Ferry Gage, and (2) there was no known
<br />reversal in the curvature of the stage-discharge rating
<br />curve at the Grand Canyon gaging station. The chief
<br />problem with this approach, however, was that unlike at
<br />Lees Ferry, where the high-water marks from the 1921
<br />nood were at known stages, there was no certainty of
<br />the peak stage of the 1921 flood at the Grand Canyon
<br />gaging station because this station was not established
<br />until November 1922. Unfortunately, Gatewood and
<br />Hunter did not correctly interpret the dates of the various
<br />high-water marks at the Grand Canyon gaging station,
<br />and mistook the likely J 884 high-water mark for the
<br />1921 high-water mark. This misinterpretation led to an
<br />overestimation of the peak discharge of the 1921 flood
<br />hy about 30 percent.
<br />Gatewood and Hunter assumed that the high-
<br />water mark found in 1933 by J,A. Baumgartner of the
<br />USGS in a left-bank recess upstream from the
<br />measurement eableway at the Grand Canyon gaging
<br />station was from the J 921 flood, By extrapolation of the
<br />stage-discharge rating curve at the Grand Canyon gaging
<br />station to the stage of this high-water mark (37.5 ft), they
<br />estimated that the peak discharge of the 1921 flood was
<br />about 220,000 ft3/s at the Grand Canyon gaging station.
<br />Because very little tributary intlow or attenuation of the
<br />peak discharge occurred between the Lees Ferry and
<br />Grand Canyon gaging stations dOling snowmelt floods,
<br />they then assumed that the peak discharge of the 1921
<br />flood at Lees Ferry was also 220,000 ft3/s,
<br />
<br />18 Computation and Analysis of the Instantaneous-Discharge Record for the Colorado River at Lees Ferry. Arizona-Mav 8, 1921, through Soptember 30, 2000
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