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<br />0;)';' ('J., <br />,. t. <br />~.J.. . j <br /> <br />13 <br /> <br />Bar graphs for each station show the water years for which the maximum <br />instantaneous discharge has been defined prior to 1957. The "water year" is <br />the 12-month period ending September 30. Thus, the 1950 water year is the <br />year that ended Sept. 30, 1950. At a few gaging stations sufficient historical <br />data were obtained to define annual peaks antedating the period of systematic <br />data collection; hence, the period of record indicated by the bar graph does <br />not always coincide with the period of record as published in the annual stream- <br />flow reports of the U. S, Geological Survey. <br /> <br />METHOD OF FLOOD-FREQUENCY ANALYSIS <br /> <br />The method of computing flood-frequency relations in this report repre- <br />sents current techniques developed in a continuing study by engineers of the <br />Geological Survey and others, Publications containing similar data for other <br />areas are listed at the end of this report. The statistical principles and anal- <br />yses outlined in the following paragraphs follow two major steps: (1) Devel- <br />opment of flood-frequency relations for a specific gaging station on a stream, <br />and (2) combining a number of gaging-station relations on an areal basis to <br />provide a means for estimating flood-frequency relations at any site, gaged or <br />ungaged. <br /> <br />Flood Frequency at a Gaging Station <br /> <br />Flood data for a gaging station may be analyzed in two ways: as an annua1- <br />flood series or as a partial-duration series, In an annual-flood series, only <br />one flood, the annual maximum, is considered for each year of record. An <br />objection to this method is that the second highest flood in a given year may <br />be greater than the maximum flood for other years. This objection is over- <br />come by use of the partial-duration series, in which all floods greater than a <br />selected magnitude or base are considered without regard to the number with- <br />in any given period, One objection to the partial-duration series is that some <br />years will have no floods above the base, Another objection to the partia1- <br />duration series is that all the floods considered may not be fully independent <br />events; that is, one flood may set the stage for another. Although for high floods <br />the recurrence intervals are practically the same for both. series, there re- <br />mains a distinctive difference in their meanings. In the annual-flood series, <br />the recurrence interval is the average interval in which a flood equal to or <br />greater than a given magnitude will recur as an annual maximum. In the partial- <br />duration series, the recurrence interval is the average interval between floods <br />of a given magnitude regardless of their relationship to the year or any other <br />period of time. The annual-flood series was used in this report, <br />