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<br />match the discharges measured at the Grand Canyon
<br />gaging station (corrected for tributary inflow) during those
<br />non-ice-affected periods when this was originally done by
<br />the USGS (see previous section).
<br />The methods used in this study deviated num those
<br />used by the USGS during the original computation of the
<br />daily mean discharge record in four respects, First, in this
<br />study, discharges at the Grand Canyon gaging station Were
<br />not used to correct any discharges at Lees FelTy on days
<br />affected by ice. All ice corrections were made during the
<br />digitizing of the stage-recorder graphs, Second, the 1938
<br />revised stage-discharge rating curve and shift curves of
<br />Gatewood and Hunter were used during the computation
<br />of discharge on the following days: June 26, 1921-
<br />September 10,1921, April 22, 1922~July 17, 1922,
<br />April 17, I 923-July 31, 1923, April 8, 1924-July 16,
<br />1924, April 1, 1925-July 21, 1925, and April 16, 1926-
<br />July 20, 1926. Uulike their method of estimatiug the peak
<br />discharges of the 1884 and 1921 floods, which was
<br />shown to be in error, Gatewood and Hunter's method
<br />of computing discharge during these six periods results
<br />in computed discharges that agree closely with those
<br />measured at the Upper Cableway. Tn fact, the discharges
<br />computed by Gatewood ,md Hunter's approach during
<br />these periods agree much more closely with the measured
<br />discharges than do the discharges computed by means of
<br />the original USGS rating curves without shifts. Third, in
<br />order to compute discharge from May 8,1921, through
<br />June 25, 1921, a smoothed stage-discharge rating curve
<br />was fit to the discharge-measurement data from water
<br />years 1921 and 1922. This rati ng curve was extended
<br />linearly from 120,000 ft3/s through the 170,000 ft3/s
<br />discharge determined in this study as the best value for the
<br />peak discharge of the June 1921 flood (fig. 13). Because
<br />oftbe uncertainty associated with the datum of the LaRue
<br />Gage, the USGS never published daily mean discharges
<br />for the period prior to June 13, 1921. The discharges
<br />computed in this study from May 8 through June 12,
<br />1921, however, arc probably reasonable because they
<br />compare favorably with both (1) the combined discharges
<br />of the upper Colorado, Green, and San Juan Rivers, and
<br />(2) the discharge of the Colorado River at the Yuma
<br />gaging station (fig, 18), Tn fig, 18, the combined daily
<br />mean discharge ofthe upper Colorado, Green, and San
<br />Juan Rivers was computed by shifting the measured daily
<br />
<br />-1:' .
<br />. ..
<br />
<br />. i,
<br />
<br />mean discharge record at the Colorudo River near Fmita
<br />gaging station +2 days, shifting the measured duily mean
<br />discharge record at the Green River gaging station + 1 day,
<br />and eSlhuating the daily mean dischurge of the San Juan
<br />River at the neur Bluff gaging station (by the same
<br />approach us in fig. 11). The daily mean discharge record
<br />at the Yuma gaging station has been shifted -9 days to
<br />account for the approximate travel time of the tlood peak
<br />between Lees Ferry and Yuma, Fourth, in order to
<br />maintain consistency in this data set, the discharges at
<br />Lees Feny during March through September, 1929, were
<br />also reduced by the amouuts stated in w.E. Dickinson's
<br />December 30, 1929, memorandum (Appendix C),
<br />
<br />200,000
<br />
<br />q
<br />z
<br />o
<br />o
<br />w
<br />~ 150,000
<br />w
<br />a.
<br />>-
<br />w
<br />W
<br />LL
<br />g 100,000
<br />OJ
<br />:::>
<br />o
<br />~
<br />w
<br />a? 50,000
<br /><(
<br />I
<br />o
<br />rn
<br />o
<br />
<br />
<br />o
<br />
<br />APRIL MAY JUNE JULY AUGUST
<br />EXPLANATION
<br />- COLORADO RIVER AT LEES FERRY
<br />,........ COMBINED UPPER COLORADO, GREEN, AND
<br />sAN JUAN RIVERS
<br />- - - COLORADO RIVER AT THE YUMA GAGING STATION
<br />
<br />_ RANGE OF UNCERTAINTY IN HIGH DISCHARGES
<br />AT THE YUMA GAGING STATION
<br />
<br />Figur.18, Comparison of the computed instantaneDus discharge for
<br />April-August 1 921 at Lees Ferry with 111 the combined daily mean
<br />discharge of the upper Colorado, Green, and San Juan Rivers, and 12) the
<br />daily mean discharge of the CDloradD River at the Yuma gaging statiDn,
<br />The gray shaded region covers the range of uncertainty in discharges in
<br />excess Df abDut 110,000 cubic feet per second at the Yuma gaging station
<br />(described above in the textl,
<br />
<br />
<br />38 Computation and Analysis of the Instantaneous-Discharge Record for the Colorado River at lees Ferrv. Arizona-May 8, 1921, through September 30. 2000
<br />
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