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<br />30 <br /> <br /> <br />the recurrence interval of the 1894 flood is substantially greater <br /> <br /> <br />than 50 years; of the 1921 flood, about 50 years; and of the 1957 <br /> <br /> <br />flood, about 25 years. These estimates are very rough and cannot <br />be used to assign discharges to those floods. Damaging floods on <br />St. Vrain Creek. at Longmont invariably have been associated with <br />heavy rains in the Plains region. There is no record of a damaging <br />flood in the study area due to snowmelt alone. <br /> <br />Outstanding floods are known to have occurred on Lefthand Creek <br /> <br /> <br />in 1864 and 1894, and there probably were floods on Lefthand Creek <br /> <br /> <br />at other times when there were noteworthy floods on St. Vrain Creek. <br /> <br /> <br />3. There is evidence of substantial recent erosion at several <br /> <br />places in the channel of St. Vrain Creek in the study area. Much of <br />this erosion occurred in May 1957. <br />4. Flood profiles for the 25- and 50-year floods along the <br />thalweg and the profile of the thalweg are shown in figure 8 for <br />St. Vrain Creek. The main channel of Lefthand Creek will contain <br />the 25-year flood between U.S. Highway 287 and the upstream limit <br />of backwater from St. Vrain Creek, and will contain the 50-year <br />flood between U.S. Highway 287 and a farm bridge about 2,000 feet <br />above the mouth. <br />5. Estimated boundaries of areaS subject to inundation by <br />the 25- and 50-year floods are shown on plate 2. Depths of inun- <br />dation by the 25-year flood can be estimated by subtracting the <br />elevation of the ground along a cross section from the elevation of <br />