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Therefore, the peak discharge of the 1921 flood at the Lees Feny should have been similar to that at the Topock gaging station, which is approximately 450 miles downstream from Lees Feny, and the Yuma gaging station, which is approximately 660 miles downstream from Lees Ferry (fig, 1A). Support for this is provided by the measured peak discharges during the high-discharge, large-volume, long-duration snowmelt floods of 1922 and 1923, which had vhtually identical peak discharges at the Lees Ferry, Topock, and Yuma gaging stations (U.S. Geological Survey, daily mean and peak discharge data, accessed November 22, 2(00). During the 1922 snowmelt flood, peak discharges at the Lees Ferry, Topock, and Yuma gaging stations were 116,000 ft3/s, 125,000 ft3/s and 117,000 It3/s, respectively; during the 1923 snowmelt flood, peak discharges at the Lees Feny, Topock, and Yuma gaging stations were 98,300 ft3/s, 1m,OOO ft3/s, and 101,OOO ft3/s, respectively. Only during lower volume snowmelt floods, with peaks of much shorter duration than during the 1921, 1922, or 1923 Hoods, was substantial attenuation of the peak discharge observed between the Lees Ferry and Yuma gaging stations. The estimated peak discharge of the 1921 Hood at the Topock gaging station was 174,000 ft3/s on June 22, 1921 (Grover and others, 1922). This peak discharge was estimated by extrapolating the stage-discharge rating curve that applied to this site in 1921. This rating curve was defined by 30 discharge measurements covering a discharge range between 8,000 and 80,000 ft3/s (Grover and others, 1922). Based on an evaluation of the discharge data at the Topock gaging station by Dickinson (1944), the 174,000 ft3/s estimate of the peak discharge may be, if anything, too high, Dickinson (1944) stated, "In general, records prior to 1 924 tend to show the discharge too great above about 20,000 second-feet [ft3/s] owing to equipment and methods then in use in making discharge measurements," In 1942, the USGS revised its estimate of the 1921 flood at the Topoek gaging station to "greater than 200,000 ft3/s" (Parker and others, 1942), but there was no basis for this revision except to make it consistent with Gatewood and Hunter's 1938 revision or the estimated peak discharge of the 1921 flood at Lees Ferry, Discharge data collected farther downstream at the Yuma gaging station during the 1921 flood are inconsistent with Gatewood and Hunter's 1938 estimate ofthe peak discharge at Lees Ferry and Parker and others' 1942 revision of the peak discharge at the Topock gaging station, The peak discharge or the 1921 flood measured at the Yuma gaging station was 188,000 ft3/s on June 27, 1921 (Grover and others, 1922), This value was never revised by the USGS and is still the accepted peak discharge at Yuma during the 1921 flood, The computed discharges at the Yuma gaging station during 1921 were based on 164 measurements made duri ng the year (Grover and others, 1922), Indeed, a discharge measurement was made on the morning of June 27, 1927, near the peak of the flood (fig, 10). The published estimate of 188,000 ft3/s at the Yuma gaging station may be slightly high because (1) the ero85- sectional area of the flow at the Yuma gaging station was determined by primitive sounding methods that would have overestimated the cross-sectional area of flow, and (2) the mean velocities in the higher flow discharge measurements during 1921 were estimated by multiplying surface velocities by a coefficient of 0.9 (Dickinson, 1944), In evaluating the discharge data at the Yuma gaging station, Dickinson (1944), stated: Priorto 1926, measurements are generally subjectto errors of varying amounts due to methods and equipment used, including the use of relatively few measuring points. During 1911-15 and at stages above low-water during 1916-22, most measurements were based on observations of surface velocity using a coefficient of 0.9 to obtain the mean. At stages above low-water, soundings were made separately from velocity observations priorto 1926, and prior to 1923 a cylindrical weight suspended from one end laxis verticall was used which was aptto be carried downstream by drift and high velocity, resulting in too large soundings. Soundings and velocity observations were further complicated by a stayline 1918-25, When the roughness elements on the bed are small relative to the How depth, velocity profiles in steady, uniform flow tend to have a logarithmic shape in only the lower 20 percent of the flow, and an approximately parabolic shape in the upper 80 percent of' the flow (Rattray and Mitsuda, 1974; Wiberg and Smith, 1991; Long and others, 1993), The coefficient relating suttaee to mean velocity in this type of' profile is not 0.9, but 0,8 (at one significant Jigure). The peak discharge of the 1921 flood at the Yuma gaging station, based on a velocity eoeflicient of 0.8 and the assumption that the sounding methods did not introduce too much error, is ]67,000 ft3/s. The peak discharge of this flood at the Yuma gaging station was therefore between 167,000 and 188,000 ft3/s; the uncertainty of this estimate is associated with the accuracy of the sounding and the cross-sectionally averaged shape of the velocity prolile. 1 ".t. ,:' , . 'N' _\ .t 24 Computation and Analvsis of the Instantaneous-Discharge Re.cord1or. the Colorado River at Lees Ferrv. Arizona-May 8. 1921. through September 30, 2000