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<br /> <br />; <br /> <br />criterion, the values of 95 percent of the measures of slope must <br /> <br /> <br />fall within limi~s that are prescribed by statistical theory. The <br /> <br /> <br />index flood commonly used by the Geological Survey is the flood with <br /> <br /> <br />a recurrence interval of Z.33 years (mean annual flood, QZ.33)' The <br /> <br /> <br />index flood is determined from a curve, a family of curves, or an <br /> <br /> <br />equation, relating values of Q2.33 to basin characteristics. The <br />index flood fixes the position of the frequency curve for a given <br />site. <br />The shape of the composite curve (fig. 1), which was computed <br />from data collected at the 12 stations listed in table 1, determined <br />the shape of the frequency curves for St. Vrain and Lefthand Creeks <br /> <br /> <br />at U.S. Highway 287 at Longmont (figs. 2 and 3). Information on <br /> <br /> <br />floods as early as 1844 was considered. Ten of the station frequency <br /> <br /> <br />curves have slopes well within the prescribed limits; the slopes of <br /> <br /> <br />the other two are only slightly outside those limits. <br /> <br /> <br />The value of Q2.33 for St. Vrain Creek at U.S. Highway 287 at <br /> <br /> <br />Longmont was computed by interpolation on a drainage area basis <br /> <br />between the areal value of Q2.33 for St. Vrain Creek at Lyons and <br /> <br /> <br />the station value of Q2.33 for St. Vrain Creek at mouth, near <br /> <br /> <br />Platteville. The value of Q2.33 for Lefthand Creek at Longmont is <br /> <br /> <br />the value of Q2.33 determined from the gaging station record for <br />Lefthand Creek. Values of Q2.33 were not correlated with basin <br />characteristics in this study because there are too few streamflow <br /> <br />records to evaluate the effects of the many variables involved. <br /> <br />9 <br /> <br />