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rr W en > >- o <l: W 0 (9 z rr <l: 0' I Z ~ 8 o W o en w rr z w ;;;: "- f- f- en W :J W en LL LL 0 o iii z :J 00 ~ ~ IT: ro :J o 250 200 150 .1984 1997. .1983 CLOSURE OF GLEN CANYON DAM .1985 .1929 . . . . . . . . . 100 . . . ... .. . . . .. . . . . . . . . . . . . . .. . . e.- ... · .-.. )...,. tit . .,. ~ .. .~it.;-:.r".~.~' :12'1 'I. . . . . . . . . . . . 50 . . . .. . . . . . o . . . . . . o N 0> ~ , ~ , ~ 0 0 0 0 0 0 0 0 '" .,. '" CD .... <Xl 0> 0 0> 0> 0> 0> 0> 0> 0> 0 ~ ~ ~ ~ ~ ~ ')J , , , , , , , ~ ~ ~ ~ ~ ~ ~ ~ , , , , ~ ~ ~ ~ ~ ~ ~ DATE Figure 38. The duratiDn Df peri Dds of sustained discharge Dver 18,500 cubic feet per secDnd (the base discharge used in the flDDd.frequency analyses), CONCLUSIONS Although its geographic location within the Colorado River drainage basin and its accessibility made Lees Fell)' a good location for a stream-gaging station, the stage dependence of the location and geometry of the hydraulic control made computations of flood discharges at this gaging station difficult during the pre-dam period, As stage increases, the water-surface profile progressively flattens at the Lees Ferry Gage as the location of hydraulic control shifts downstream. This development of backwatered flow conditions causes a reversal in the curvature of the stage-discharge rating curve, such that, at stages in excess of about 15 ft, discharge is proportional (0 stage raised to a power of less than one. This reversal in curvature made cxtrapolation of the stage-discharge rating curve problematic; thus there was considerable uncertainty in the estimated peak discharges of the largest historical floods at Lees Ferry in 1884 and 1921. The USGS originally estimated that these two floods had peak discharges of about 210,000-250,000 and 174,000 ft3/s, respectively. On the basis of a key assumption now known 56 Computation and Analysis of the Instantanoous-Discharge Record for tho Colorado River at Lees Ferry, Arizona-May 8. 1921. through September 30, 2000