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Table 2. Stage-discharge rating curves used to construct tile published record of daily mean discharge at the Lees Ferry gaging station during water years 1921 through 1986 Rating curve name Dates used Rating Dates used curve name 10-15-29 10-01-1928 through 5-29-1929 10-18-29 5-30-1929 through 9-30-1929 10-15-32 3-20-1932 through 6-12-1933, 6-21-1933 through 9-30-1934 11-9-33 6-13-1933 through 6-20-1933 3-0-36 10-01-1934 through 9-30-1936 1-6-38 10-01-1930 through 2-28-1938 1-14-39 3-01-1938 through 3-20-1939 9-5-39 3-21-1939 through 10-31-1939 9-10-40 11-01-1939through 1-20-1943 H-27-43 1-21-1943 through 4-24-1948, 1-01-1949 through 4-23-1949 7-23-48 4-25-1948 through 12-3]-1948 7-25-49 4-24-1949 through 4-30-1952. 1-01-]953 through 12-31-1955 8-5-52 5-01-1952 through 12-31-1952 RT] 1-01-1956 through 5-31-1963 RT2 6-01-1963 through 9-30-1968 RT3 10-01-1968 through 9-30-1972 RT4 10-01-1972 through 4-30-1983 RT5 5-01-1983 through 10-18-1984 RT6 10-19-1984 through 9-30-1980 '5-03-38 1]-12-23 11-16-23 11-20-23 1-08-24 10-30-24 2-04-20 2-15-26 12-06-26 12-09-26 12-08-26 2-24-26 12-14-27 12-15-27 12-10-28 12-11-28 6-13-1921 through 7-10.1921 ,8-23-1921, 5-21-1922 through 6-2-1922, 5-01-1923 through 5-31-1923 7-11-1921Ihrough8-22-1921, 8-24-1921 through 9-30-1921 10-01-1921 through 5-20-1922, 6-03-1922 through 7-10-1922 7-1 1-1922 through 1-18-1923 1-19-1923 through 4-30-1923, 6-01-1923 through 9.30-1923 10-01-1923 through 9-30-1924 10-01-1924 through 9-19-1925 9-20-1925 through 9-30-1925 10-01-1925 through 5-06-1926, noon on 2-17-1927 through 4-20-1927 5-07-1926throogh 5.30-]926 5-31-1926 through 12-16-]926 12-17-1926 through noon on 2-17-1927 4-21-1927 through 7.01-]927 7-02-1927 through 9-30-1927, 5-15-1928 throogh 9-30-1928 10-01-1927 through 2-29-1928 3-01-1928 through 5-14-1928 11938 revision by Gatewood and Hunter. discharge, which would then bc used to compute a daily mean discharge, After the closure of Glen Canyon Dam, when the discharge each day became bighly variable due to hydroeleclric-power generalion, the discharge record for all days would be computed by this subdivision method, Since May 7, ] 926, shifts have been applied to the measured stage prior to computing the discharge with a rating curve (table 3), This standard USGS method is known as the "shifting-control" method (described on page 25 in Kennedy, 1984), Shift curves for each year wcrc bascd on the smoothed disagreement between the measured discharge and the discharge predicted by the stage-discharge rating curve. For example, if, over a given period, the observed slages during dischargc measurements were consistenlly 0,1 foot higher than the 'stages associated with those dischargcs on the rating curve, 0, I foot would be subtracted from the measured stages for that period prior 10 computing dischargc with the stage-discharge rating curve. Thus, use of thc shifting- conlrol melhod at Lees Ferry allowed the published discharges to agree more closely with thc mcasurcd discharges. Glen Canyon Darn has greatly reduced the annual range of water temperature in the river at Lecs FelT)', Prior to elosure of the darn, ice routinely made the computation of discharge diflicull at Lees Ferry during thc winter (figs, 1M-B). During extremely cold pcriods, the river would completely freeze over, thc float would fi'ceze in the well, and no stage record would be obtained, More typically, the river would freeze at night, and thc stage would increase slowly. The ice would then begin to melt the next morning, and the stage would decrease quickly around noon (fig. l6C), Other types of ice effect at the I 1 1 1 1 1 I I . , . , . .. , \ , , , 34 Computation and Analysis of the Instantaneous-Discharge Record for the Colorado River at Lees Ferry. Arizona-May 8, 1921, through September 30, 2000