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<br />2.3 Principal Floou Problems <br /> <br />As noted earlier, most of the annual precipitation in the Yampa River <br />Basin occurs as snow and a deep snowpack accumulates in the high <br />elevations. General rainstorms covering large areas for extended <br />periods can occur in the Steamboat Springs region from late spring <br />through early fall. Convective cloudburst storms of small areal <br />extent can be expected frequently during the summer. <br /> <br />Major floods in Steamboat Springs have been the result of snowmelt <br />augmented by rain in spring or early summer. In general, snowmelt in <br />spring or early summer constitutes a frequent but comparatively minor <br />flood threat. General rainstorms alone have not caused significant <br />floods in the Yampa River Basin. However, there is no reason to <br />conclude that rare but very large floods resulting from general <br />rain are not possible in the Yampa River Basin. Records show that <br />major flood producing rainstorms almost always occur during the <br />months of September and October in western Colorado. Precipitation <br />records for Steamboat Springs show that a storm in October 1925, <br />produced 1.13 inches of rain over a 2-day period, and that a storm in <br />September 1939, produced 1.64 inches in a 3-day period. The highest <br />runoff of record on the Yampa Rive0 in June 1921, was the result of <br />heavy rain falling on a melting snowpack. There is little definitive <br />information available on flooding from cloudburst storms in the <br />Steamboat Springs area. The storm that caused flooding in the city <br />in 1921 may have been a cloudburst. This storm produced more than <br />2.5 inches of rain, but its duration is conjectural. A maximum of <br />2.04 inches in 3 hours has been recorded elsewhere in the region. <br />Thus, flooding from cloudburst storms is a probability in the Steam- <br />boat Springs area. <br /> <br />Some areas in Steamboat Springs are subject to sheetflowi that is, <br />broad, shallow, overland flooding generally less than 3 feet deep and <br />characterized by unpredictable flow paths. The water-surface eleva- <br />tions of flooding in these areas are essentially independent of those <br />along adjacent strearnways and are affected principally by obstruction~ <br />in the flooded areas. <br /> <br />Analyses done for this report show that the lOO-year flood on the <br />streams under study would result from snowmelt augmented by rain, <br />that the SOO-year flood on the Yampa River would result from general <br />rain, and that the SOQ-year flood on the tributary creeks would <br />result from cloudburst storms. <br /> <br />The worst flooding known in the Steamboat Springs area occurred in <br />June 1921, and April 1974. Flooding was widespread in the region in <br />1921, with highway travel cut off and towns isolated. In Steamboat <br />Springs, flow in Soda Creek was too great to be passed by the Lincoln <br />Avenue crossing, and a large lake for.med that surrounded several <br />homes and was backed up by the bridge. Floodwater ultimately flowed <br />over the street in a stream which was a half block wide. The bridge <br /> <br />13 <br />