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<br />; <br /> <br />From figure 2, the SO-year floo4 (2-percent chance of being <br />exceeded in any given year) on St. Vrain Creek at U.S. Highway 287 <br />at Longmont is 10,800 cfs (cubic feet per second); the 25-year <br />flood (4-percent chance) is 5,800 cfs. From figure 3, the 50-year <br />flood on Lefthand Creek at U.S. Highway 287 at Longmont is 1,840 cfs; <br />the 25-year flood is 1,000 cfs. For the purpose of this study, it <br />was assumed that the 50- and 25-year floods on Lefthand Creek will <br />occur at the same time as the corresponding floods on St. Vrain Creek <br />and consequently, the 50- and 2S-year floods on St. Vrain Creek below <br />the mouth of Lefthand Creek are estimated as 12?600 cfs and 6,800 cfs, <br />respectively. Such concurrence is unlikely, but alternative assump- <br />tions would not have changed the results shown appreciably. <br />The flow in Pry Creek and the other small tributaries will in- <br />crease the flow in St. Vrain Creek somewhat, but there are insufficient <br />data to determine the magnitude and frequency of floods on very small <br />streams in the Plains region. However, consideration of the magnitudes <br />of the 50- and 25- year floods on Lefthand Creek indicates that the <br />effect of the flood-flow contributions of small tributaries in the <br />study area will be negligible. <br />FLOOP HISTORY <br />U. S. Geological Survey Water-Supply Paper 997, "Floods in <br />Colorado" (Follansbee and Sawyer, 1948) contains accounts of floods <br />and associated meteorological events before about 1940 on streams in <br />Colorado, including St. Vrain Creek. In some cases the accounts <br /> <br />10 <br /> <br />