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<br />6 <br /> <br /> <br />i <br /> <br />. <br /> <br />at extremely high elevations where streams have characteristics that <br />differ substantially from those for streams at lower elevations. <br />Gaging station records used in this report are listed in table 1 <br />(p. 7). The locations of all the gaging stations except one are <br />shown on plate 1. The station on Cache 1a Poudre River near Greeley <br />is about six miles downstream from the eastern edge of the area <br />shown on plate 1. <br />Information concerning historical floods contained in Geological <br />Survey Water-Supply Papers and files of local newspapers is supple- <br />mented by photographs taken during outstanding floods and interviews <br />with local residents. <br />Engineers of the Geological Survey obtained the field data <br />necessary to determine the hydraulic characteristics of the stream <br /> <br />valleys and their constrictions. <br /> <br />MAGNITUDE AND FREQUENCY OF FLOODS <br />The chance of occurrence of a given flood is generally described <br />in terms of either its probability of occurrence or its recurrence <br />interval. Recurrence interval, used in this report, is defined as <br /> <br /> <br />the average interval of time within which a given flood will be <br /> <br /> <br />equaled or exceeded once. Inference should not be made that the <br /> <br /> <br />"50-year flood", for example, will occur once, and only once, during <br /> <br />a 50-year period. It may occur in any year, or even in successive <br /> <br /> <br />years. In terms of probability, a 50-year flood has 1 chance in 50, <br /> <br /> <br />or a 2-percent chance, of being equaled or exceeded in any given <br /> <br />. <br />