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FLOOD03777
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Last modified
1/25/2010 6:28:14 PM
Creation date
10/5/2006 12:03:51 AM
Metadata
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Template:
Floodplain Documents
County
Larimer
Community
Greeley
Stream Name
Cache La Poudre
Basin
South Platte
Title
Prehistoric Floods in Poudre Canyon
Date
2/1/1992
Prepared For
State of Colorado
Prepared By
CSU
Floodplain - Doc Type
Flood Documentation Report
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<br />Slackwater deposits have successfully been used to <br />reconstruct floods on rivers in the united states, Australia, <br />India, Israel, China, and South Africa. <br /> <br />V. RESULTS <br /> <br />Slackwater deposits occur in several locations in the Poudre <br />basin. We have identified them in lower Poudre Canyon near Upper <br />picnic Rock, above and below the Big Narrows, above the Home <br />Moraine, and along the lower South Fork of the poudre (Figure 1). <br />We have examined the deposits at the lower Poudre and lower South <br />Fork sites, where the stratigraphy of the flood sediments <br />indicates multiple floods. <br /> <br />We have defined a 2-km-Iong study reach along Poudre Canyon <br />in the vicinity of Upper picnic Rock based on four slackwater <br />depositional sites. During the autumn of 1990 and the spring of <br />1991 we surveyed 23 channel and valley cross sections within this <br />reach, and used the HEC-2 program to model flood discharges along <br />it. simulated flood discharges ranged from 530,000 to 3500 ft3s- <br />1 (Figure 2). The surveying was done with a laser theodolite, <br />which is accurate to within 5 mm, and we believe that these flow <br />simulations provide accurate minimum estimates of peak flood <br />discharges. <br /> <br />These discharge values are significantly larger than the <br />highest gaged discharges at the canyon mouth gaging station, <br />which has been in operation since 1882. The peak discharge <br />during this period of record was 21,000 ft3s- , in June 1891. <br />Three radiocarbon ages for the slackwater sediments in lower <br />Poudre Canyon indicate an age range of 700 to 6500 years before <br />present (Figure 3). <br /> <br />We have also conducted a less detailed examination of flood <br />sediments in the lower canyon of the South Fork of the poudre. <br />Here, two flood units are preserved in a small cave along the <br />right bank of the channel. A slope-area estimation of peak <br />discharge based on three surveyed cross sections produces a <br />discharge of 120,500 ft3s-1, which is again si~nificantly higher <br />than the peak recorded discharge of 1260 ft3s- at the Rustic <br />gage, which is upstream of the study site, and has been operated <br />since 1957. A radiocarbon age from bone fragments found in the <br />South Fork cave is pending. <br /> <br />VI. SUMMARY <br /> <br />The findings summarized in this report may well be <br />significant for flood frequency analysis, dam and reservoir <br />construction, and downstream floodplain zoning. However, in <br />order to evaluate the potential significance of the paleoflood <br />data in terms of predicting future floods, it is necessary to <br />develop a more thorough chronology of past floods, and to <br />identify the probable causes of past floods. There are two <br />potential causes of large prehistoric floods in Poudre Canyon; <br />
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