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0 70,000 z 0 A. () w CLOSURE OF GLEN CANYON DAM <n ~ 60,000 w ~ ~ w DAM OPERATIONS w ~ 50,000 CONSTRAINED () " ON AUGUST 1,1991 ~ () " 40,000 I ~ z 0 ~ 30,000 I () ~ w i IIII~ w Ij " ~ ~ I IIII () <n I 11I11 11M )11 ~lilll~\,I~#1i ~Jv, 1/ is I \ !'~IIIIII I ~ ~ \,,~ ~ z 0 ~ 0 0 0 0 0 0 ~ ~ w , w m = m '" '" m m ~ 0 200,000 z 0 B. () w w ~ w CLOSURE OF GLEN CANYON DAM ~ ~ 150,000 w ~ DAM OPERATIONS () ro CONSTRAINED ~ ON AUGUST 1,1991 () " I ~ 100,000 z 0 ~ I () ~ W " 50,000 ~ ~ I () <n is ~ ~ ~ ~ 0 , 0 0 0 0 0 0 0 ill ~ ~ w , w m = m '" '" m ~ ~ ~ ~ DATE Figure 35. The minimum and maximum discharges of the Colorado River at Lees Ferry each month from May 1921-September2000, (AI Minimum discharge each month, 181 Maximum discharge each month, Flood Frequency During the Pre-Dam Period To evaluate the natural frequency of floods on the Colorado River at Lees Ferry, partial-duration and annual flood-frequency analyses were conducted on the pre-dam part of the continuous record of instantaneous~isch:irge (fig. 36). To extend these analyses to larger floods with longer return periods, the 1884 flood and the paleoflood data of O'Connor and others (1994) were also includcd in these analyses. The base discharge selccted for the partial-duration analysis was thc instantaneous discharge between May 8, 1921, and March 12, 1963, that was equaled or exceeded only 25 percent of thc time, 18,500 ft3/s (fig. 22A), This basc discharge was also chosen because it was cxceeded about 25 perccnt of thc time during both the prc- and post-dam periods of record, In this section of the paper, thc term "flood" is used to describe any discharge above this base discharge. During the pre-dam period between May 8, 1921, and March 12, 1963, 277 floods occurred with peak discharges in cxcess of 18,500 ft3/s, The return periods for the pre-dam flood data were computed on the basis of different lengths of record, The return periods for thc 277 floods between May 8, 1921, and March 12, 1963, werc computed on the basis of their 41.8-year period of record, Because the 1884 flood had the largest peak discharge of any flood at Lces Ferry between June 1884 and March 1963, the return interval for the 1884 flood was computcd on thc basis of a 78,2-ycar period of record, The return pcriod of the largcst paleoflood identified by O'Connor and others (1994), the -300,000 ft3/s paleoflood that lcft behind the crevice deposit, was recomputed on the basis ofthe 1,200- 1,600-ycar age of the deposit rather than the 2,307-year period of record used by O'Connor and others (1994). Inclusion of the data of O'Connor and others (1994) in this flood-frequency analysis first required a reevaluation of the return periods for thcse palcofloods. 0' Connor and others assumed that the 10 paleofloods which left the deposits in Axehandle Alcove that are less than 2,062-2,307 years old (including the historic tlood that produced dcposit "G 1 ") werc equally spaced over 2,307 ycars, This assumption led O'Connor and others to compute retmn periods of 200 to 800 years for the eight paleofloods associated with the dcposits underlying "G 1." During the carly part of the 20th century, howcver, floods observed at the Yuma gaging station on the Colorado River had peak discharges equivalent to those of thcse eight prehistoric paleotloods. Because the natural Junc tributary int10w to the Colorado River between Lees Fcny and Yuma was minimal, the peak discharges of thesc floods at Lees Ferry and Yuma should have been comparable, Between 1903 and 1920, five snowmelt floods at the Yuma gaging station had peak discharges that ranged from either 120,000 to l70,000 ft3/s (when a coefficient of 0,8 is uscd to relate the measurcd surface velocities to the mean velocities, as explained previously) or 135,000 to 190,000 ft3/s (when a coefticient of 0.9 is used), 1 1 I , 52 Computation and Analysis of the Instantaneous-Discharge Record for the Colorado River at lees Ferry. Arizona-May 8, 1921. through September 30, 2000