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<br />years of record, and its magnitude has an estimated frequency <br />of occurrence of once in approximately 60 years_ <br /> <br />The river in the problem area is characterized by low banks, <br />braided channels, and a considerable amount of gravel, <br />cobbles, and snags deposited on gravel bars. <br /> <br />The river carries a large bedload of gravel and cobbles, some <br />of the latter being more than 6 inches in diameter. The <br />capacity of the channel has been reduced by this sediment. <br />In some instances, where channel changes took place during <br />the flood, the original channels were so filled with sediment <br />that nearly all of the present flows are discharging through <br />the new channels. Generally, however, the new channels are <br />old watercourses abandoned by the river in previous years. <br />The littered condition of the channels, and the increased <br />danger of bank erosion, inundationl and resultant channel <br />changes constitute the present flood problem. <br /> <br />No urban areas are affected by floods. The principal items <br />damaged by the 1957 flood were the agricultural lands and <br />roads and bridges adjacent to, or over the river. A few farm <br />buildings were flooded. The total known damages in all <br />categories in this reach amounted to slightly more than <br />$45,000. <br /> <br />A corr~on type of damage from the 1957 flood was bank erosion. <br />The slope of the stream ranges from 65 feet per mile above <br />Basalt to 40 feet per mile below Basalt. Velocities probably <br />in excess of 10 feet per second occurred during the 1957 <br />flood. The banks are composed largely of sand, gravel, and <br />cobbles overlain by a comparatively thin mantle of soil, and <br />are quite erosible. Spoil-type dikes, constructed by local <br />interests with material bulldozed in the process of channel <br />clearing, proved to be qery erosible~ <br /> <br />A substantial percentage of the total damages were due to <br />overtopping of the low banks and the inundation of pasture <br />and croplands. The banks in most instances range from only 2 <br />to 5 feet in height. In 2 locations, where old channel areas <br />have been reclaimed, considerable volumes of overflow and <br />widespread flooding resulted from bank overtopping at the <br />upper ends of the areas. <br /> <br />More than one-half of the known damages in the 1957 flood <br />were caused to roads and bridges in the area, with nearly <br />$24,000 of damages being caused to 3 bridges and their <br />abutments. Two of these were public bridges and have since <br />been restored. A third bridge, which was private, was <br />completely destroyed and has not been replaced. <br /> <br />5 <br />