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<br />discharge was greater than 9,000 ft3/s 62.2 percent of the
<br />time; during the] 980s, discharge was greater than
<br />9,000 ft3/s 75,5 percent of the time; and during the ]990s,
<br />discharge was greater than 9,000 ft3/s 82.6 percent
<br />of thc time, Through time, the post-dam river has been
<br />progressively dominated by discharges that erode sand,
<br />and base flows have disappcared, Dischargcs most likely
<br />to crode sand from Marble and upper Graud Canyons
<br />OCCUlTed during the 1990s.
<br />Dam operations for power generation have
<br />increased the median daily range in c1ischarge by a factor
<br />of 15,8 re]ativc to the pre-dam median dai Iy range in
<br />discharge. Thc post-dam median daily range in discharge,
<br />8,580 it3/s, actually exceeded the pre-dmn median
<br />discharge 01'7,980 ft3/s, As a result of this increase in thc
<br />sub-daily variability in discharge, daily mean discharges
<br />no longer provide an adequate characterization of the
<br />hydrology of the Colorado River. The decade with the
<br />largest daily range in discharge was the reservoir-filling
<br />decade of thc 1970s, when the median daily range in
<br />discharge was 13,700 113/s (a factor of 25.2 greater than
<br />the pre-dam median daily range in dischm'ge), The decade
<br />with the smallest daily range in dischm'ge was the 1990s,
<br />whcn the median daily range in discharge was 4,940 ft3/s
<br />(a factor of9,I greater than the pre-dam median daily
<br />range in discharge), Rclative to the pre-dam period of
<br />record, dam operations for hydroelectric-power
<br />generation have increased the daily range in dischargc
<br />during all but 0,1 percent of all days. Prior to closure of
<br />the dam, the daily discharge range exceeded lO,OOO it3/s
<br />on only about I perccnt of all days, During the post-dam
<br />period, the daily discharge range exceededlO,OOO fl3/s on
<br />about 43 percent of all days,
<br />The operations of G]en Canyon Dam also have
<br />altered the frequency of floods on the Colorado River at
<br />Lees FelTY. Prior to closure of the dam, the avcrage
<br />recurrencc intervals were 1 year for floods with peak
<br />discharges of 50,000 ft3/s, and 6 years for floods with
<br />peak discharges of 120,000 ft3/s. Although dam
<br />operations have maintained the pre-dam frequency of
<br />floods with peak discharges of abont 29,000 ft3/s, the
<br />frequency of floods with peak discharges greater than this
<br />valne has been reduced, wh<:oreas the frequency of floods
<br />with peak discharges ]ess than this va]ne has been
<br />increased, For example, the 2-year flood during the pre-
<br />dam period was 85,000 ft3/s whereas the 2-year flood
<br />during the post-dam period was 3] ,500 ft3/s, Because of
<br />this increase in the frequency of smaller "floods," the
<br />annualllood series is inadequate to characterize the flood-
<br />frequency distribution in the post-dam river. During the
<br />
<br />pre-dam period of record, the avcrage recurrence interval
<br />for floods with peak dischm'ges of 20,000 ft3/s was 97
<br />days. After closure of the dam, these 20,000-ft3/s floods
<br />increased in frequency by a factor 01'27, so that their
<br />recurrence interval is now 3,6 days. Tn addition to
<br />increasing the frequency of these lower floods, operation
<br />of the dam has also resulted in the longest periods of
<br />sustained high discharge; all four of the longest periods of
<br />sustained discharge in excess of 18,500 ft3/s occurred
<br />after c10surc of the dam,
<br />
<br />REFERENCES CITED
<br />
<br />Bureau of Reclamation, J 990, Glen Canyon Environmental
<br />Studies, Area 2, River Mite -4.0 to +2,0, Contract No, 8-
<br />CS-40-0527B D008: Rapid City, S.D" Horizons, Inc"
<br />Scale 1:2400, 5 sheets.
<br />_2000, Lower Colorado Region
<br />Law of the River, accessed February I, 2003, at URL
<br />hltp://www.1c.usbr.gov/gIOOO~awofrvr.htm I
<br />Burkham, D.E., 1986, Trends in selected hydraulic variahles for
<br />the Colorado River at Lees Perry and near Grand Canyon,
<br />Adzona-1922-1984: Glen Canyon Environmental
<br />Studies Report No, PB88-2 I 6098, 58 p.
<br />Chow, V.T., 1959, Open-channel hydraulics: New York,
<br />McGraw-Hill, 680 p.
<br />Colby, BR, 1964, Scour and fill in sand-bed slreams: U,S.
<br />Geological Survey Professional Paper 462-D, 32 p.
<br />Dickinson, W.E., 1944, Summary of records of surface waters
<br />at base stations in Colorado River Basin, 1891-1938: U.S,
<br />Geological Sorvey Water-Supply Paper 918, 274 p,
<br />Ferrari, R.L., 1988, 1987 Lake Powell Survey: Bureao of
<br />Reclamation technical rcpOlt REC-ERC-88-6, 67 p.
<br />Fradkin, P.L" 1984, A river no more: The Colorado River and
<br />the West: Tucson, Ariz., The University of Arizona Press,
<br />360 p,
<br />Grover, N,C., Follansbee, R, Purlbn, A.B,. and Rice, RC.,
<br />1922, Surface water sopply of the United States, 1921, Part
<br />IX, Colorado River Basin: U.S. Geological Survey Water-
<br />Supply Paper 529, 181 p.
<br />_' 923, Surface water supply of the Uuited States, 1922,
<br />Part IX. Colorado River Basin: U.S. Geological Survey
<br />Water-Supply Paper 549, 258 p.
<br />Grover, N.C., Dickinsou, W.E., Follansbee, R, Gardiner, J,H.,
<br />Johuson, B., aud Purton, A,B" 1939, SurFace water supply
<br />of the United States, 1938, Part IX, Colorado River Basiu:
<br />U.S. Geological Survey Water-Supply Paper 859, 285 p.
<br />Hereford, R" Burke, K..l., and Thompson, K.S" 2000, Map
<br />showing Quaternary geology and geomorphology or the
<br />Lees Ferry area, Arizona: U.S. Geological Survey
<br />Geologic Investigations Series 1-2663,
<br />
<br />
<br />58 Computation and Analvsis of the Instantaneous-Discharge Record for the Colorado River at Lees Ferry. Arizona-Mav 8, 1921, through September 30.2000
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