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On the basis of fig, 4A, the water-surface elevation dUling the peak of the 1884 flood at Axehandle Alcove was probably about 3,136.5 ft above sea level, 6.6 ft higher than deposit "G L" The highest deposit in Axehandle Alcove, the 1,200-1,600 year-old crevice deposit, is about 16.4 ft higher than deposit "GI," and about 9,8 ft higher than the likely peak stage of the 1884 flood. Given that the water sUlface was probably almost flat between the site of the Lees FelTY Gage and Axehandle Alcove (fig. 4A), the peak stage of this 1,200-1,600 year-old flood at the site of the Lees Ferry Gage would have been about 41 ft, Extrapolation of the stage-discharge rating curve in fig, 13, therefore, suggests that the peak discharge of the paleoflood that deposited the crevice deposit was about 300,000 ft3/s. Therefore, based on these revised paleoflood discharges and the stratigraphy of O'Connor and others (1994), the deposits of 15 paleofloods during the last 4,500 years with peak discharges larger than 120,000 ft3/s are preserved at Axehandle Alcove, Of these paleoflood deposits, 10 are preserved from floods during the last 2,100-2,300 years with peak discharges in excess of about 140,000-150,000 ft3/s, and one deposit is preserved from a flood 1,200-1,600 years ago with a peak discharge in excess of about 300,000 ft3/s. COMPUTATION OFTHE CONTINUOUS RECORD OF INSTANTANEOUS DISCHARGE AT LEES FERRY FOR WATER YEARS 1921-2000 All of the raw data collected by the USGS at Lees Ferry were rel1ieved from storage in the Federal Records Centers (Appendix A), After this retrieval, computation of the continuous record of instantaneous discharge was a multi-step process that took almost 4 years to complete. Methods Used by the U,S. Geological Survey to Compute the Previously Published Daily Mean Discharge Record for Water Years 1921--86 The first step in this process was to review the methods used by the USGS to compute the previously published daily mean discharge record. DUling water years 1921-86, the USGS computed discharge at the Lees Feny gaging station by the standard method of using a stage-discharge rating curve to convert measured stage to discharge (Rantz and others, 1982), Prior to January 19, 1923, stage was observed twice daily at the vmious staff gages in the reach between the Upper Cableway and the Lees Ferry Gage, After this date, stage was measured continuously by the float in the stilling well and was recorded on an analog Sl1ip-chart recorder. The chart recorder was replaced as the primary record of stage on February 2, 1967, when a digital recorder was installed in the gage, After this date, the analog chart record was used only as a back-up in case the digital recorder failed. From Februmy 2,1967, through February 28,1974, stage was recorded digitally eveIY 2 hours, Subsequently, stage was recorded digitally every 30 minutes to increase the accuracy of the published daily mean discharges. During each water year, between 5 and 200 discharge measurements were made (fig. 15), and used to construct the stage-discharge rating curves and to compute shift curves. Thirty-six different rating curves were used to compute the published record at Lees Ferry horn June 13, 1921, through September 30, 1986 (table 2). During most days in the pre-dam era, daily mean discharge was computed by applying a stage-dischm'ge rating curve to a daily mean stage. During days when the discharge was variable, the day would be first subdivided into portions of compmable stage. Then, the mean stage for each of these pOltions would be convelted into a mean 200 if! >- Z Wo: 2< Ww 0: >-150 :00: if!w <>- w< 2$ ~I 0: ~100 ~w u0 if!Z 00: LL:O 00 [[ w 50 wO ",< 22 :0 Z o 1920 1930 1940 1950 1960 1970 1980 1990 WATER YEAR Figure 15. Number of discharge measurements made at Lees Ferry during each water year between 1921 and 1986, 02235 Computation of the Continuous Record of Instantaneous Discharge at Lees Ferry for Water Years 1921-2000 33