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<br />23 <br /> <br />. <br /> <br />during large floods to which this figure can be compared, However, <br /> <br />the U.S. Army Corps of Engineers, Omaha Engineer District (1971, <br /> <br />p. 15) predic ts a floo~ of 44,000 c. f. s. on the Bi'g Thompson River <br />(at Loveland) as the practical upper limit of flooding, Larger <br /> <br />floods could occur but would be extremely rare. At these locations <br /> <br />the Cache la Poudre River drains three ti~es the area of the Big <br /> <br />Thompson River (Table 1). Thus, 63,000 'c.f.s, does not seeman <br /> <br />unreasonable discharge for a large flood on the Cache la Poudre <br /> <br />River. <br /> <br />The calculated flood discharges on the Big Thompson and South <br /> <br />Platte Rivers were extremely large; 128,000 c.f.s, 1 7 percent and <br /> <br />177 , 000 ~,f. s, 1 13 percent respectively. There have, been no gaged, <br /> <br />historic floods that approach this magnitude~ <br /> <br />Accuracy of Nagnitude and Frequency Evaluations <br /> <br />. <br /> <br />The evaluation of flood magnitude and frequency depends on <br /> <br />the accuracy of the historic record, eyewitness reports and the <br /> <br />calculations of discharge, There is no reason'to believe that the <br /> <br />historic record :and eyewitness reports are inaccurate. <br /> <br />The accuracy of calculated discharges depends primarily.on <br /> <br />the cross section that is d.rawn. Figure 6 illustrates variations <br /> <br />in calculated discharges ~ntroduced when the cross section is not <br /> <br />well controlled with field data. Variations from a mean for such <br /> <br />calculations are very large. The variations are greatly reduced <br /> <br />when a more precise cross section can be d~awn. <br /> <br />Changes in land use may increase the magnitude and frequency <br />