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<br />loumal of Geology
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<br />14000
<br />13000
<br />~12000
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<br />~7000
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<br />2000
<br />1000
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<br />LARGE FLOODS ON THE COLORADO RIVER
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<br />3
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<br />-One Flood 1600-1200 yr BP (350-750 AD)
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<br />
<br />Stratigraphic Record
<br />at
<br />Axehandle Alcove
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<br />Ten Floods during last 2000-2300 yr BP
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<br />
<br />> 15 Floods during last 4500 yr BP
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<br />1884 Historic Flood
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<br />-- 1921
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<br />Gaged Record.
<br />Colorado River at
<br />Lees Ferry, Arizona
<br />Closure of
<br />Glen Canyon
<br />I Dam
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<br />1880
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<br />1900 1920
<br />Year (AD)
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<br />1940
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<br />1960
<br />
<br />1980
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<br />Fipue 1. Measured and historic annual peak discharges of the Colorado River at Lees Ferry, and a summary of the
<br />stratigraphic record of floods, 3 km downstream, at Axehandle Alcove,
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<br />mJsec-1 and the peak stage of the 1921 flood las
<br />noted above, the discharge was calculated at the
<br />Grand Canyon gage) however, peak stage was mea-
<br />sured at both gages) to the reconstructed stage of
<br />the 1884 flow (Dickinson 1944, p. 174), Therefore,
<br />the 1884 discharge estimate must be considered
<br />only an approximate one. The 1884 flow is thought
<br />to be the largest since at least 1868 (USGS 1973,
<br />p, 6111, Flow has been regulated by the Glen Can-
<br />yon Dam ~ince the beginn;ng of 1963, Flow records
<br />at the Lees Ferry gage are considered "excellent,"
<br />meaning that 95% of the daily discharges are
<br />within 5% of true values (USGS 1973, p, 7), The
<br />accuracy of the annual peak discharges, which are
<br />the values used in the flood frequency analysis that
<br />follows, is uncertain, but probably is somewhat
<br />less, Dickinson (1944, p, 174) stated that records
<br />for flows greater than 4250 mJsec -I (only the 1921
<br />floodl are "fair," indicating an accuracy of :t 15%,
<br />Flow records at other Colorado River gages pro-
<br />vide little additional reliable information of large
<br />floods, Mean monthly flows at Lees Ferry for the
<br />period 1911-1921 have been estimated by laRue
<br />(1925), and revised by Dickinson (1944, p, 175-
<br />1771, from records at upstream gages, No mean
<br />monthly discharges in this period exceeded the
<br />
<br />mean monthly discharge of the 1921 flood, al-
<br />though a large June, 1917 discharge may have had
<br />a peak discharge close to the 6250 m3sec -1 peak
<br />discharge of 1921. At Yuma, Arizona, Colorado
<br />River stage has been recorded since 1878, and dis-
<br />charges have been measured since January, 1903
<br />(Dickinson 1944, p. 244-255), Gaged discharges of
<br />5400 m3sec-1 and 5300 m3sec-1 were recorded in
<br />June, 1920 and June, 1921, respectively. Larger dis-
<br />charges at Yuma occurred some years during win-
<br />ter, due primarily to flood flows on the Gila River,
<br />In the period 1878-1902, the highest measured
<br />stage was during the Colorado River flood of 1884,
<br />except for January, 1891, when an exceptional
<br />flood on the Gila River resulted in a higher stage at
<br />Yuma, Near Topock, Arizona (figure 11. Colorado
<br />River discharges have been measured since 1917,
<br />Measurements before 1924, however, are consid-
<br />ered unreliable IDickinson 1944, p, 201). Observa-
<br />tions of highwater marks near the site of the To-
<br />pock gage indicate the a pre-1884 flood achieved a
<br />stage 2,6 m higher than the 1884 flood, Dickinson
<br />11944, p, 2011 estimated that this earlier discharge
<br />exceeded 11,300 mJsec-l, and "that it probably oc-
<br />curred within the period 1857-68 and most likely
<br />in 1862,"
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