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<br />ARTICLES
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<br />A 4500- Year Record of Large Floods on the Colorado River in the
<br />Grand Canyon, Arizona I
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<br />Jim E, O'Connor, Lisa L Ely2 Ellen E, WohP, Lawrence E. Stevens4
<br />Theodore S. Melis5, Vishwas S, Kale6, and Victor R. Baker
<br />v,s, Geological Survey, 5400 MacArthur Blvd. Vancouver WA, 98661
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<br />ABSTRACT
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<br />A sequence of flood deposits left by the Colorado River in the Grand Canyon, Arizona, provides evidence of at least
<br />IS floods with peak discharges greater than 5500 m3see-1 over the last 4500 yr, Ten floods during the last 2000-
<br />2300 yr had discharges greater than 6800 m3see-I, One flood, 1600-1200 yr ago, had a discharge exceeding 14,000
<br />m3sec -I, a flow rate more than twice the largest gaged flood, This record of flooding is one of the longest for a major
<br />U,S, river, and, combined with the gaged record of twentieth century floods, allows determination of the frequency
<br />and history of large floods that have affected key aspects of Colorado River geomorphology,
<br />
<br />"When the summer sun comes this snow melts and
<br />tumbles down the mountain sides in millions of cas-
<br />cades, A million cascade brooks unite to form balf a
<br />bundred rivers beset with cataracts: balf a bundred
<br />roaring rivers unite to form the Colorado which rolls, a
<br />mad, turbid stream, into the Gulf of California,"
<br />Powell (18751
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<br />Introduction
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<br />Before construction of Glen Canyon Dam in the
<br />early 1960s, large Colorado River floods were an
<br />integral element of the geomorphology, hydrology,
<br />and ecology of the Grand Canyon. Consequently,
<br />many features of the present Colorado River envi-
<br />ronment reflect the passage of large floods, For ex-
<br />ample, most of the famous Grand Canyon rapids
<br />are the result of bauldery debris fans deposited by
<br />debris flows in steep tributary canyons (Cooley et
<br />al, 1977; Howard and Dolan 1981; Webb et al,
<br />1988a, 1989), Only extremely large main-stem
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<br />I Manuscript received March I, 19931 accepted June 15,
<br />1993,
<br />1 E.arth System Science Center, pennsylvania State Univer.
<br />aity, Univeroity Park PA 16802,
<br />J Dept. Earth Resources, Colorado State University, Fan
<br />Collina, CO 80523,
<br />. National Park Service, P,O, Box 22459, Flagstaff AZ 86002,
<br />5 Dept. Geosciences, University of Arizona, Tucson AZ
<br />85721-
<br />· Depl, Geography, Univeroily 01 Poona, Pune-411 007lndia,
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<br />floods were competent to transport the large boul-
<br />ders contained in these debris fans lera! 1979,
<br />Kieffer 1985), AIl a result, the present distribution
<br />and size of sand bars and river rapids, features of
<br />significant recreational value, are largely the cu-
<br />mulative result of the sequence of interactions be-
<br />tween side canyon debris flows and past Colorado
<br />River floods (Howard and Dolan 1981i Kieffer
<br />1985; Webb et al, 19911. Likewise, large floods
<br />played an important role in the development of
<br />clearly defined vegetation zones along the Colo-
<br />rado River IStevens 1989), Flood-induced germina-
<br />tion and mortality, deposition of suitable sub-
<br />strates, and moisture availability substantially
<br />influenced the distribution of long lived riparian
<br />plant species IClover and Jotter 1944; Stevens
<br />1989; Johnson 19911,
<br />Determining the past sequence of large flows
<br />and quantifying the magnitude and frequency of
<br />floods large enough to have affected some of these
<br />aspects of the Colorado River has been difficult.
<br />The gaged record, while long 11921 to present) rela-
<br />tive to gaged records on western U.S, rivers, is not
<br />long enough to address satisfactorily the issues
<br />described above, Researchers investigating long-
<br />time-scale processes such as channel-and-rapid
<br />evolution have had to rely on measurements of the
<br />largest historic floods IGra! 1979}, or indirectly es-
<br />timate the largest prehistoric floods on the basis of
<br />present channel morphology and a model of river
<br />hydraulics and particle erosion (Kieffer 1985), or
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<br />[The Journal of Geology, 1994, vol~ 102, p. 1-91 No copyright claimed for this article,l' <emaina in the poblle donWn, 0022-1376/94/1020HlO3S1.00
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