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Table 3, Oates when the shifting-control method was used to construct the published record of daily mean discharge at the Lees Ferry gaging station during water years 1921 through 1986 Date 5-07-1926 through 5-8-1926 5-31-1926 through 6-16-1926 40-8-1928 through 4-22-1928 5-07-1928 through 5-10-1928 5-15-1928 through 6-12-1933 6-21-1933 through 10-12-1936 10-28-1936 through 11-14~1936 12-24-1936 through 7-18-1937 8-03-1937 through 9-30-1940 5-17-1941 through 5-19-1941 5-30-194] through9-30-1968 12-14-1968 through 9-30-1973 6-04-]983 through 6-27-1983 8-03-1983 through 10-18-[984 5-09-1986 through 9-30-1986 Lees Ferry Gage included: (I) the formation oflarge ice jams on the riffle at the Paria River gravel bar that would back up large quantities of water and cause rises in stage by a foot or more, until the ice jams failed, and (2) the complete freezing of the surface of the river, during which the float would be frozen in the stilling well. DUling periods when the Lees Ferry Gage was affected by ice, daily mean discharge was typically computed by subtracting the sum of the measured and estimated inflows of water between the Lees Ferry and the Grand Canyon gaging stations from the daily mean discharge computed at the Grand Canyon gaging station. Between 1931 and 1945, the record at the Grand Canyon gaging station was routinely used to modify the discharge computed at Lees Ferry dUling periods of higher flow in the spring and summer (Dickinson, 1944). During higher flows, slightly more water was sometimes computed passing Lees Ferry than was computed passing the Grand Canyon gaging station, This artifact was largely due to instability in the Lees Ferry stage-discharge rating curve above the reversal in curvature at a stage of 15 ft, Beginning in water year 1931, the discharge record at the Grand Canyon gaging station (because of its more stable stage-discharge rating curve) was used to reduce the computed discharges at Lees Ferry in order to bring them into agreement with those computed at the Grand Canyon gaging station (fig, 17), Although this was done almost every year from 1931 through 1945, these reductions were the most extensive during water years 1934 and 1936, During water year 1934, the daily mean discharges computed at Lees Ferry were reduced by I percent from May 9 through May 28, During water year 1936, the daily mean discharges computed at the gage were reduced by I percent from May 20 through June] 5,2 percent from June ]6 through June 20,3 percent from June 21 through June 30, 2 percent from July I through July 4, and I percent from July 5 through July 25, Prior to 1931, it was not the policy of the USGS to use the record at the Grand Canyon gaging station to modify the discharge record at Lees Ferry for periods longer than a few days, except during periods of ice at Lees FelTY. The one extended period prior to 1931 when the record at the Grand Canyon gaging station should have been used to modify the disc barges at the Lees Ferry gaging station was water year ]929, Between March 6 and September 30, 1929,2,7 percent more water was computed passing Lees Ferry than was computed passing the Grand Canyon gaging station (after conections had been made for the tributary inflow of water between the two sites), In an unpublished USGS memorandum dated December 30, 1929, WE. Dickinson computed that the discharges at the Lees Ferry gaging station were 2.6 percent too high from March 6 through March 2], 3.3 percent too high from April 2 through April 15, 3.4 percent too high from Aprill5 though May 2, 2.7 percent too high from May 2 through July 16,3,5 percent too high from July 16 through August 26, 4.9 percent too high from August 26 through September 19, and 2.3 percent too high from September 19 through September 30 (Appendix C). Although the cause of the discharge en'ors at the Lees FelTY gaging station between March and September 1929 were never officially determined, there are three possible explanations. The Upper Cableway was replaced in March 1929, and this could have affected the discharge measurements. The cunent meters used at Lees Ferry were changed on February 15, and this change in cunent meters could also have affected the discharge measurements. Finally, in March 1929, the abandoned dugway road on the left side of the gage was filled with rocks (Appendix B. fig, B6), This till prevented water from flowing around both sides of the gage house during high discharges and could have affected the stage measurements at the gage, (\I)I')~" . l_ 0-. . ~ Computation of the Continuous Record of Instantaneous Discharge at Lees Ferry for Water Years 1921-2000 35