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<br />Therefore, the combined discharge of the upper Colorado,
<br />Green, and San Juan Rivers contributing to the peak
<br />discharge of the June 1921 flood at Lees Ferry was
<br />approximately 167,000 fi3/s.
<br />
<br />Revised Estimate of the Peak Discharge of the
<br />1921 Flood at Lees Ferry
<br />
<br />Because the peak discharge of the Jnne 1921 flood
<br />had to be the same throughout the Lees FelT)' reach,
<br />extrapolations of the stage-discharge rating curves at the
<br />Lees Ferry Gage and the three staff gages in the l-mile-
<br />long reach upstream were used to establish constraints on
<br />the peak discharge of this flood (fig. 12), A strong reversal
<br />in curvature is evident in the stage-discharge rating curves
<br />defined by the data from the Lees Feny, Number 4, and
<br />Cable Gages, and a mild reversal in curvature is evident in
<br />the stage-discharge rating curve defined by the data from
<br />the Number 1 Gage, At stages in excess of those at which
<br />the reversal in curvature occurs at these four gages,
<br />
<br />analyses of the goodness of fit oflinear and power-law
<br />regressions indicate that the high-stage parts of the
<br />stage-discharge rating curves at these gages are best
<br />approximated by either a linear or a power-law relation,
<br />in which discharge is proportional to stage raised to a
<br />power of less than one (fig, 12).
<br />The reversals in curvature of the stage-discharge
<br />data at the Lees Ferry Gage for the entire pre-dam period
<br />of 1921-{j2 and for only water-years 1921-22 both
<br />occur at a stage of 15 ft. Linear regressions fit to the
<br />1921-{j2 and 1921-22 data above this stage are virtually
<br />identical (fig, 12A), and have R2 values of 0.929 and
<br />0,932, respectively. Similarly, power-law regressions
<br />fit to the 1921-{j2 and 1921-22 data above this stage are
<br />also virtually identical (fig. 12A), and have R2 values of
<br />0.932 and 0.974, respectively. The similarity of the
<br />regressions fit to the 1921-{j2 and the 1921-22 data
<br />indicates that the upper end of the stage-discharge rating
<br />curve at the Lees Ferry Gage was fairly stable over time,
<br />
<br />A. 240,000 8,240,000
<br /> 220,000 PUBLISHED PEAK DISCHARG 220,000 PUBLISHED PEAK DiSCHARGE
<br />0 OF THE 1921 FLOOD 0 OF THE 1921 FLOOD
<br />z 200,000 Z 200,000
<br />0 0
<br />() ()
<br />w 180,000 w 180,000
<br />U) U)
<br />a: a:
<br />w 160,000 w 160,000
<br />Q. Q.
<br />t- REVERSAL IN Iii REVERSAL IN
<br />w 140,000 CURVATURE 140,000 CURVATURE
<br />w OF DATA w OF DATA
<br />u. w u.
<br />() 120,000 I I () 120,000
<br />1ii t- W 1ii W
<br /> u.D I J:
<br />::> 100,000 00 t- ::> 100,000 t-
<br />() w9 u. () u.O
<br />~ COu. 0 ~ 00
<br />ui 80,000 e'> w ui 80,000 wO
<br />co U)S! '" '" ",IT'
<br /> ;:': <!-
<br />a: 60,000 "'- a: 60,000 t;~
<br /><! <! U) <!
<br />a: w '" I ,,-
<br />() 40,000 Q. <! 0 40,000 <!
<br />U) W U) W
<br />0 Q. 0 Q.
<br /> 20,000 20,000
<br /> 0 0
<br /> 5 10 15 20 25 30 35 40 5 10 15 20 25 30 35 40
<br /> STAGE AT LEES FERRY GAGE, IN FEET STAGE AT NUMBER 4 GAGE, IN FEET
<br />
<br />EXPLANATION
<br />
<br />o 1921--B2 DATA
<br />
<br />- LINEAR AND POWER-LAW REGRESSIONS FIT TO ALL
<br />THE 1921-62 DATA ABOVE A STAGE OF 15 FEET
<br />
<br />,,--~ "". V~ LINEAR AND POWER-LAW REGRESSIONS FIT TO ONLY
<br />THE 1921-22 DATA ABOVE A STAGE OF 15 FEET
<br />
<br />
<br />
<br />EXPLANATION
<br />
<br />o 1921-23 DATA
<br />
<br />- LINEAR AND POWER-LAW REGRESSIONS FIT TO THE
<br />1921-23 DATA ABOVE A STAGE OF 16 FEET
<br />
<br />Figur. 12. Linear and power-law extrapolations of the stage-discharge rating curves at the Lees Ferry, Number 4, Cable. and Number 1 Gages,
<br />{AI Rating curve at the lees Ferry Gage defined by 1he 4,114 non-ice.affected discharge measurements made during water years 1921-62, Stage is rela1ive
<br />to the Dugway Gage and Lees Ferry Gage datum of Sep1ember 21, 19211see Appendix BI, Shown for comparison are the peak stages of the 1921 and
<br />1884 floods and the published peak discharge of1he 1921 flood,lb1 Ra1ing curve at1he Number 4 Gage defined by the 209 discharge measurements made
<br />during water years 1921-231hat included stage observations at the Number 4 Gage, Stage is relative to the Number 4 Gage datum of September 29. 1921
<br />[see Appendix BI, Shown for comparison are 1he peak s18ge and published peak discharge of the 1921 flood, Stage is relative to the Number 1 Gage datum
<br />of September 26, 1921 {see Appendix 81. Shown forcomparison are the peak stage and published peak discharge olthe 1921 flood,
<br />
<br />'O(1).38
<br />. '."'. Colorado River floods at Lees Ferry 27
<br />
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