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<br />INTRODUCTION <br /> <br />Methods of determining flood-frequency relations, for simplicity, can <br />be grouped into two general types. One consists of using streamflow- <br />gaging station records; the other uses rainfall-runoff relations. In <br />many parts of the United States, flood relations from these two methods <br />yield comparable results. <br /> <br />In the method based on streamflow records, the annual flood series is <br /> <br />analyzed statistically to obtain flood magnitudes at selected recurrence <br /> <br /> <br />intervals using guidelines proposed by the Interagency Advisory Committee <br /> <br /> <br />on Water Data (1981). Because streamflow records are collected at only a <br /> <br /> <br />few of the many sites where information is needed, streamflow-gaging <br /> <br /> <br />station information must be transferred to ungaged sites. Regional <br /> <br /> <br />analysis is concerned with extending records spatially and provides a tool <br /> <br /> <br />for regionalizing streamflow characteristics (Riggs, 1973). In addition, <br /> <br /> <br />regional analysis may produce improved estimates of streamflow charac~er- <br /> <br /> <br />is tics at the gaged sites by reducing time-sampling errors. Multiple <br /> <br />regression is used to relate the discharge for a given frequency to <br /> <br />climatic, basin, and channel-geometry characteristics, leaving residuals <br /> <br />that may be considered due to chance. The regression line averages these <br /> <br />residuals, In Colorado, several regional analysis reports are available to <br /> <br />estimate flood-frequency relations (McCain and Jarrett, 1976; Livingston, <br />1981; and Kircher et a1., 1985). <br /> <br />~ <br />