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<br />the Little Thompson River flood plain, respectively, as described <br />in this report. Interstate Highway 25 presents the largest poten- <br />tial for obstructing floodflows on the Big Thompson River. County <br />roads at reference points 47, 59, 82, and 103, the Colorado and <br />Southern Railway, and U. S. Highway 287 present the largest flood- <br />flow obstruction potential on the Little Thompson River. <br /> <br />Manmade objects, such as buildings, cars, and fences, as <br />well as trees, brush, and other vegetation located on the flood <br />plain, can also cause floodflow obstructions. These items, while <br />remaining in place, not only tend to create higher stages on the <br />flood plain because of reduced flow area and flow blockage but may <br />also create higher localized velocities as the floodwaters flow <br />around them. Flood stages may be lowered in a localized area, if <br />the floodflows are of sufficient magnitude to dislodge these flow <br />obstructions. The debris may then iodge at locations farther down- <br />stream, thus compounding problems. This floating debris commoniy <br />lodges against bridge piers and bridge decks; the flow area Is then <br />reduced or the channei is completely blocked. The blockage can <br />produce severe stage and velocity Increases, often accompanied by <br />bridge damage or complete failure. <br /> <br />Ice effects have, historically, not been a problem on <br />the Big Thompson and Little Thompson Rivers. Ice jams, however, <br />could occur in the future, although the chance of occurrence of <br />such a phenomenon is quite remote. <br /> <br />Flood damaqe reduction measures - Olympus Dam, constructed <br />in 1949 by the Bureau of Reclamation, is located near Estes Park in <br />the upper Big Thompson River basin. This structure, however, has <br />negligible effect in reducing flood damages on the reach of the <br />Big Thompson River studied In this report because of the relatively <br /> <br />13 <br />