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<br />100,000 <br /> <br />A. <br /> <br />10,000 <br /> <br />1,000 <br /> <br /> <br />Q <br />Z <br />o <br />o <br />w <br />w <br />cr <br />w <br />~ <br />~ <br />w <br />~ <br />o <br />in <br />~ <br />o <br />g <br />ui <br />o <br />cr <br />~ <br />" <br />&5 100,000 <br />6 <br />g <br />w <br />o <br />z <br />~ <br />oc <br />OJ <br />~ <br />Q <br /> <br />100 <br /> <br />10 <br /> <br />MONTHLY MEDIAN DAILY RANGE IN DISCHARGE <br />_______ PRE-DAM <br />-0- POST-DAM <br /> <br />1 <br />JAN FEB. MAR. APR. MAY JUNE JULY AUG. SEPT. OCT. NOV. DEC. <br /> <br />B, <br />,,-,,'A' ...L'! . 6-.-.::::... Z: <br />......--.-... <br /> <br />... <br />:~::: ::l'i "--'6.m::~::'.:.^.. <br />':.. <br /> <br />10,000 <br /> <br />...-..... <br /> <br />MONTHLY MAXIMUM DAILY RANGE IN DISCHARGE <br /> <br />----..... PRE-DAM <br /> <br />....A-.. POST-DAM <br /> <br />1,000 <br /> <br />MONTHLY MINIMUM DAILY RANGE IN DISCHARGE: <br /> <br />100 <br /> <br />_______ PRE-DAM <br /> <br />-0- POST-DAM <br /> <br /> <br />10 <br /> <br />FEB. MAR. APR. MAY JUNE JULY AUG. SEPT. OCT. NOV, DEC. <br />MONTH <br /> <br />Figure 33. The effects of dam operations on: (AI Monthly median daily ranges in <br />the discharge of the Colorado River at lees Ferry, (6) Monthly minimum and <br />maximum daily ranges in the discharge of the Colorado River at Lees Ferry. Daily <br />ranges in discharge plotted equal to 1 indicate that the minimum daily range in <br />discharge during these months was less than the level of detection. <br /> <br />Although the post-dam duration of low discharges decreased <br />after closure of the dam (figs, 22A and 29), monthly minimum <br />discharges during the post-dam period prior to August I, <br />1991, were actually lower than the monthly minimum <br />discharges during the pre-dam months of Februmy through <br /> <br />70,000 <br /> <br />Q <br />Z <br />o <br />&":60,000 <br />w <br />cr <br />w <br />~ <br />tii 50,000 <br />w <br />~ <br />o <br />m <br />[540,000 <br />g <br />ui <br />" <br />~ 30,000 <br />" <br />o <br />w <br />6 <br />~ 20,000 <br />w <br />o <br />z <br />~ <br />0: 10,000 <br />OJ <br />~ <br />Q <br /> <br />- PRE-DAM <br />(May9,1921-March12,1963) <br /> <br />.....-... POST-DAM <br />(March 14, 1 963-September 30, 2000) <br /> <br /> <br />o <br /> <br />o 0 ;; <br />o 0 <br />o <br /> <br />UJOOOOOOOUJ 0'> <br />..... <\IC') U') 1'--000 CF>CF> 0'> <br /> <br />rn 0 rn <br />~ ~ $ <br />g: <br /> <br />PERCENTAGE OF DAYS WITH RANGE IN DISCHARGE <br />LESS THAN VALUE SHOWN <br /> <br />Figure 34. Probability graph of the daily range in the dIScharge of the <br />Colorado River at Lees Ferry during the pre- and post-dam periods of record. <br />Operation of Glen Canyon Dam has greatly increased the daily range in <br />discharge during most days, <br /> <br />November (figs. 30B and 35A), After August I, 1991, <br />discharges lower than 5,000 ft3/s could not be released <br />from the dam at any time (fig. 35A) and dischm'ges <br />lower than 8,000 ft3/s could be released from the dam <br />only at night (U.S, Department of the Interior, 1995; <br />National Research Council, 1996). Because of the <br />slightly wetter weather pattems in the Upper Basin <br />during the mid- to late-1990s, a full reservoir, and these <br />new constraints on dam operations, discharges lower <br />than the pre-dam median discharge were no longer a <br />substantial part of the hydrology of the Colorado River <br />downstream from Glen Canyon Dam by the late 1990s. <br />Likewise, by the late I 990s, discharges lower than <br />the maximum pre-dam discharge at which sand <br />accumulated in Marble and upper Grand Canyons <br />(9,000 ft3/s) were no longer a substantial part of the <br />hydrology of the Colorado River downstream from <br />Glen Canyon Dam, <br /> <br />02226 <br /> <br />Analysis of the Continuous Record of Instantaneous DiSCharge 51 <br />