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<br /> <br />It seemS unlikely that the channel of Lefthand Creek will be <br /> <br />altered sufficiently to impair its capacity seriously. However, if <br />there is major deterioration of its hydraulic characteristics such <br />as heavy growth of vegetation, extensive flooding will occur. <br />SUMMARY <br />The results of the hydrologic and hydraulic analysis that is <br />the basis of this report can be summarized as follows: <br />1. Magnitudes of flood discharges for St. Vrain and Lefthand <br />Creeks at U.S. Highway 287 at Longmont for various recurrence in- <br /> <br /> <br />tervals are given by the curves in figures 2 and 3. The 50-year <br /> <br /> <br />(2-percent chance of occurring in any given year) flood on St. <br /> <br /> <br />Vrain Creek at U.S. Highway 287 is 10,800 cfs; the 25-year (4-per- <br /> <br /> <br />cent chance) flood is 5,800 cfs. The 50-year flood on Lefthand <br /> <br /> <br />Creek at U.S. Highway 287 is 1,840 cfs; the 25-year flood is 1,000 cfs. <br /> <br /> <br />The 50-year flood on St. Vrain Creek below the mouth of Lefthand <br /> <br /> <br />Creek is estimated as 12,600 cfs; the 25-year flood as 6,800 cfs. <br /> <br /> <br />2. Floods on St. Vrain Creek at Longmont that were outstanding <br /> <br /> <br />enough to warrant attention by historians or contemporary newspaper <br /> <br /> <br />accounts occurred in the spring of 1844, June 1864, May 1876, May 31, <br /> <br /> <br />1894, June 1, 1914, June 1921, June 1949, August 1951, May 1957, and <br /> <br /> <br />May 1958. The flood of May 31, 1894, was the greatest flood since at <br /> <br /> <br />least 1876, and it probably exceeded the floods of 1876, 1864, and <br /> <br /> <br />1844. Based on pictorial evidence and computations made to deter- <br /> <br /> <br />mine the hydraulic capacity of St. Vrain Creek, it is estimated that <br /> <br />29 <br /> <br />