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To make the stage-discharge rating curve at the Lees Ferry Gage pass through a discharge of 220,000 ft3/s at the measured peak stage of the 1921 flood (26.5 ft), Gatewood and Hunter imposed a second reversal in curvature at a stage of 17 ft (fig, 7). As shown in a preceding section of this paper, the water-smface slope at the Lees Ferry Gage decreases with increasing stage as a result of the progressive development of back watered flow conditions in the reach. When water-surface profiles progressively flatten with increasing stage, discharge is proportional to stage raised to a power less than one. Beginning in water year 1939, the USGS adopted the 1938 assumption of Gatewood and Hunter that there is a second reversal in the curvature of the Lees Ferry stage- discharge rating curve at a stage of about 17 ft. Thus, they assumed that, at discharges above about 85,000 ft3 Is, discharge returned to being proportional to stage raised to a power greater than one, Dickinson (1944) reiterated this assumption, stating that a "moderate reversal in the station rating curve occurs in the range from 60,000 to 80,000 second-feet [ft3/s], above and below which rating has normal curvature." Though this assumption was made, it was not supported by the data (fig. 8). Analysis of the data from the six highest flow years at Lees FelTY provide no support for the existence of this second reversal in curvature, No second reversal in curvatnre is evident in any of the individual years with substantial data above a stage of 17 ft (water years 1921-22, 1928, 1941, 1949,1952, and 1957, fig. 8). An earlier USGS memorandum written by G,c. Stevens on May 25,1925, suggests that the more likely peak stage of the 1921 flood atthe Grand Canyon gaging station was 33 ft, not 37.5 ft. As stated in this memorandum, on September 13, 1923, J.W, Johnson, resident USGS hydrographer at the Grand Canyon gaging station, wrote that the highest visible high-water mark on the cliff on the south side of the river at the Grand Canyon gaging station was at a stage of about 32 ft, This high- water mark was at a much higher stage than the peak stage of any flood at this gaging station since its establishment in November 1922. On March 14, 1924, G,G, Sykes of the USGS used a level to detennine that the stage of this high- water mark was 33 ft (figs. 9A-C), Because it was much higher than the peak stage of any flood on the Colorado River after June 1921, the high-water mark at a stage of 33 ft was probably from the June 1921 flood. - These data from the early 1920s suggest that the high-water mark at a stage of 37.5 ft found in 1933 at the Grand Canyon gaging station was probably from the 1884 flood, and not from the 1921 flood. The 1884 flood was the largest historical flood prior to 1921, and the peak stage of this flood was about 4 ft higher than that of the 1921 flood at Lees Ferry, If the high-water mark at a stage of 37.5 ft was from the 1921 flood, then the high-water mark at a stage of 33 ft should have been erased by the 1921 flood, and should not have been prominent in photographs taken at the site in November 1921 (figs, 9A- B). Power-law extrapolation of the stage-discharge rating curve fit to the pre-dam data from the Grand Canyon gaging station suggests that. at a stage of 33 ft, the peak discharge of the 1921 flood would be about 163,000 ft3/s, and, at a stage of 37.5 ft, the peak discharge of the 1884 flood would be about 214,000 ft3 Is. Extrapolation along this same curve suggests that the peak discharge of a prehistoric flood associated with the highest high-water marks found near the gage by the National Park Service during constlUction of the Kaibab Bridge would be in excess of 300,000 ft3/s, and possibly as high as about 360,000 ft3/s (fig. 9D). Other Measurements of the Peak Discharge of the 1921 Flood Made Downstream and Upstream from Lees Ferry Peak discharges measured at gaging stations downstream and upstream from Lees Ferry during the June 1921 flood indicate that the original USGS estimate of the peak discharge of the June 1921 flood at Lees FelT)' of 174,000 ft3/s was probably cOlTect, because these other measurements range between 167,000 and 188,000 ft3/s, None of the observations made at these other gaging stations indicate that the discharge was as high as the 220,000 ft3/s value estimated by Gatewood and Hunter in 1938. Comparison with measurements made at downstream gaging stations is valid because snowmelt floods on the tributaries entering the Colorado River downstream from Lees Ferry would have peaked earlier than late June. Thus, there would have been very little inflow from these tributaries to increase the peak discharge substantially, Furthermore, because the volume of water within the 1921 snowmelt flood was large and the duration of this flood was long, there would have been very little attenuation in the peak discharge of this flood between Lees Ferry and downstream gaging stations, 02242 Colorado River floods at Lees Ferry 19