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<br /> <br />; <br /> <br />year. Similarly, a 25-year flood has a 4-percent chance of being <br />equaled or exceeded in any given year. Most hydrologists agree <br />that it is impossible to determine the magnitude of the greatest <br />flood that can possibly occur on a given stream. <br />The method used by the Geological Survey in flood-frequency <br />analyses is outlined in detail by Dalrymple (1960). Briefly, the <br />hypotheses are, (1) that there are fairly large regions within <br />which the shapes of frequency curves for sites on streams within <br />the region are similar, and (2) that there are fairly large areas <br />(not necessarily coinciding with the regions of hypothesis 1) <br />within which the position of frequency curves for individual sites <br />can be related to measurable basin characteristics. The curves or <br /> <br />equations expressing the relations implied in the two hypotheses <br /> <br /> <br />are deduced from records of peak discharges collected within the <br /> <br /> <br />area and from measures 9f basin characteristics that can be made <br /> <br /> <br />from topographic, geologic, or soils maps, or from field surveys. <br /> <br />The relation that determines the shape of frequency curves <br /> <br />within a region is expressed by a curve known as a composite frequency <br /> <br />curve that relates flood events, expressed as dimensionless ratios <br /> <br />to a selected index flood, to recurrence interval. The criterion <br /> <br />for delineating the region in which the composite curve applies is <br />that the variation of the values of a measure of the slopes of the <br /> <br />frequency curves computed from data for stations in the region is <br /> <br />not greater than can be attributed to chance alone. To meet this <br /> <br />. <br /> <br />8 <br /> <br />