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<br />the dam has also largely eliminated discharges during <br />which sand could be demonstrated to accumulate <br />in the river. In addition to radically changing the <br />hydrology of the river, operation of the dam for <br />hydroelectric-power generation has introduced large <br />daily fluctuations in discharge. During the pre-dam <br />era, the median daily range in discharge was only <br />542 ft3/s, although daily ranges in discharge <br />exceeding 20,000 ft3/s were observed during the <br />summer thunderstorm season, Relative to the pre- <br />dam period of record, dam operations have increased <br />the daily range in discharge during all but 0, I percent <br />of all days. The post-dam median daily range in <br />discharge, 8,580 ft3/s, exceeds the pre-dam median <br />discharge of 7,980 ft3/s. Operation of the dam has <br />also radically changed the frequency of floods on <br />the Colorado River at Lees Ferry. The frequency <br />of floods with peak discharges larger than about <br />29,000 ft3/s has greatly decreased, while the <br />frequency of smaller 1100ds, with peak discharges <br />between 18,500 and 29,000 ft3/s, has increased <br />substantially. Operation of the dam has greatly <br />extended the duration of smaller 1100ds; for example, <br />each of the four longest periods of sustained 110ws in <br />excess of 18,500 ft3/s occurred after closure of the <br />dam, <br /> <br />INTRODUCTION <br /> <br />Lees Feny on the Colorado River was initially <br />chosen hy the U.S. Geological Survey (USGS) as a <br />measurement site because it was strategically located with <br />respect to the hydrology of the Colorado River drainage <br />basin and was readily accessible (Rusho and Crampton, <br />1992; Reilly, 1999). Lees Feny was readily accessible by <br />automobile in the I 920s, and was the first point where the <br />combined runoff could be measured from the upper part <br />of the Colorado River drainage basin, which includes the <br />upper Colorado, Green, and San Juan Rivers (fig. IA), <br />Lees FelTY also was chosen as the location for a gaging <br />station because it was several mi les downstream from a <br />proposed dam site iu lower Glen Canyon favored by the <br />Southern California Edison COIl'pany;the cooperator who <br />maintained this gaging station for the first several years. <br /> <br />Stage of the Colorado River at Lees Ferry, Arizona, <br />has been measured since May 8, 1921, when the first staff <br />gage was installed by E,C, LaRue of the USGS, <br />Subsequently, stage was read at least twice daily on <br />several or more staff gages installed in the Lees Feny <br />reach. Discharge bas been measured at Lees FelTY since <br />August 3, 1921, when the first cableway across the river <br />was completed, Stage of the Colorado River at Lees Ferry <br />has been measured continuously since JanuaJY 19, 1923, <br />when a permanent recording stage gage, consisting of a <br />strip-chart recorder connected to a float assembly housed <br />in a concrete stilling well, became operationaL This <br />recording stage gage is the modern gage and is located in <br />Glen Canyon National Recreation Area, 15,5 miles <br />downstream from Glen Canyon Dam, and I mile <br />upstream 1rom the mouth of the Paria River and the <br />northeastern boundaty of Grand Canyon National Park <br />(fig, I B). In this paper, the recording stage gage housed in <br />the concrete stilling well is referred to as the Lees Ferry <br />Gage. The historical staff gages in the reach (fig. I C) and <br />the Lees FelTY Gage together are referred to as the Lees <br />Ferry gaging station, The official USGS station names and <br />numbers for the Lees Feny and other gaging stations used <br />in this study, and the shOltened names used in this paper <br />arc listed in table I, <br />The Lees Ferry gaging station has been <br />continuously maintained since 1921 because the site is <br />also strategically located in a political sense. One year <br />arter the establishment of this gaging station, the 1922 <br />Colorado River Compact was negotiated between the <br />seven states in the Colorado River drainage basin, The <br />Compact divided the drainage basin into two parts: the <br />Upper Basin and the Lower Basin (lig, IA). In the <br />Compact, the dividing point between the basins was <br />delined as a "point in the main stream of the Colorado <br />River one mile below the mouth of the Paria River," Thns, <br />the Lees Ferry gaging station and another gaging station <br />installed in November 1923 on the Paria River became the <br />measurement points nsed to determine compliance with <br />the terms of the Compact. Under the Compact, the United <br />States-Mexico Water Treaty of 1944, and the Colorado <br />River Basin Project Act of 1968, 8.25 million acre-feet of <br />water must pass to the Lower Basin each year, of which <br />8.23 million acre-feet mnst pass the Lees FelTY gaging <br />station (June 8, 1970, Criteria for coordinated long-range <br />operation of Colorado River reservoirs pursuant to the <br />Colorado River Basin Project Act of September 30, (968). <br /> <br />I <br />I <br />l <br />i <br />~ <br />i <br />~ <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />, <br />I <br />I <br />I <br />I <br />I <br />~ <br />I <br />I <br />I <br />I <br />I <br />I <br />, <br />I <br />I <br />I <br />I <br />l <br />( <br />I <br />( <br />i <br />( <br />I <br />I <br />( <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />l <br />I <br />, <br />, <br />I <br />I <br />I <br />I <br />I <br />~ <br />i <br />. <br />I <br />l <br />I <br />I <br /> <br />2 Computation and Analvsis of the Instantaneous-Discharge Record forthe Colorado River at lees Ferry, Arizona-May 8, 1921, through September 30, 2000 <br />