<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).
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<br />2 Computation and Analvsis of the Instantaneous-Discharge Record forthe Colorado River at lees Ferry, Arizona-May 8, 1921, through September 30, 2000
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