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<br />JAN. FEB. MAR. APR. MAY JUNE JULY AUG. SEPT. OCT. NOV. DEC.
<br />
<br />MONTH
<br />EXPLANATION
<br />_______ PRE-DAM MONTHLY MEDIAN DAILY RANGE IN DISCHARGE
<br />-----/:}.---- PRE-DAM MONTHLY MINIMUM DAILY RANGE IN DISCHARGE
<br />- D - PRE-DAM MONTHLY MAXIMUM DAilY RANGE IN DISCHARGE
<br />
<br />Figure 26. Pre-dam monthly median, minimum, and maximum daily ranges in
<br />the discharge of the Colorado River at Lees Ferry. Daily ranges in discharge
<br />plotted equal to 1 indicate that the minimum daily range in discharge during
<br />these months was less than the level of detection.
<br />
<br />dam, and were referred to informally by Bureau of
<br />Reclamation engineers as "channel cleaning flows"
<br />(Grams and others, Utah State University, written
<br />comruun., 2002), During these 3 months of high
<br />discharge, approximately 5,0 million tons of fine sediment
<br />(that is, sand and finer material) were scoured from Glen
<br />Canyon between the dam and Lees Ferry (computed on
<br />the basis of the USGS daily suspended-sediment data
<br />from the Lees Ferry gaging station; U.S. Geological
<br />Survey, accessed November 15, 2000), and approximately
<br />l7,6 million tons of fine sediment were scoured from the
<br />reach between the Lees Ferry and Grand Canyon gaging
<br />stations (Rubin and Topping, 2001), From July 1965 to
<br />June 6, 1980, the dam was operated in such a way
<br />as to fill Lake Powell reservoir gradually, to fulfill
<br />downstream water-delivery obligations, and to maximize
<br />the generation of hydroelectric power. As shown in this
<br />study, this "reservoir-filling peliod" was the period
<br />of the greatest daily fluctuations in discharge for power
<br />generation (fig. 28). From the initial filling of the reservoir
<br />on June 6, 1980, until August I, 1991, the dam was
<br />operated as it was prior to 1980, with the additional safety
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<br />PERCENTAGE OF DAYS WITH RANGE IN DISCHARGE
<br />LESS THAN VALUE SHOWN
<br />
<br />EXPLANATION
<br />-PRE-DAM ALL (May 9, 1921-March 12, 1963)
<br />- - PRE-DAM JULY THROUGH OCTOBER (summer thunderstorm season)
<br />...on PRE-DAM NOVEMBER THROUGH JUNE
<br />
<br />Figure 27. Probability graph of the pre-dam daily range in the discharge of the
<br />Colorado River at Lees Ferry. Daily ranges in discharge prior to the closure of
<br />Glen Canyon Dam were the most extreme during the summer thunderstorm
<br />season (July through Octoberj,
<br />
<br />constraint of drawing down the reservoir early in the
<br />year to provide enough volume for the snowmelt flood,
<br />This period included the largest post-dam flood, the
<br />97,000 ft3/s June 1983 flood (Martin, 1989), After this
<br />flood, dam operations were revised to release more water
<br />earlier in the spring to reduce the frequency of spills of
<br />this magnitude (National Research Council, 1996), Since
<br />August I, 1991, dam releases have been constrained in an
<br />attempt to minimize the downstream effects of the dam on
<br />the Colorado River ecosystem in Glen Canyon National
<br />Recreation Area and Grand Canyon National Park
<br />(National Research Council, 1996), Under these
<br />constraints, (I) discharges no lower than 5,000 ft3/s could
<br />be released, (2) discharges no lower than 8,000 ft3/s could
<br />be released during the day, (3) discharges no greater than
<br />20,000 ft3/s could be released (this upper limit was
<br />increased to 25,000 ft3/s under the 1996 Record of
<br />Decision signed by the Secretary of the Interior), and
<br />(4) additional limits were placed on the daily range in
<br />discharge and the rates at which di~ilrge could change
<br />(ramping rates). 02 2 ~ 15
<br />
<br />Analvsis of the Continuous Record of Instantaneous Discharge 47
<br />
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