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St. Vrain Creek roughly parallels the Burlington Northern Railroad east of Second Avenue and flows through open land that is zoned medium-density residential. 2.3 Principal Flood Problems ~, Floods in the study area of St. Vrain Creek usually occur during May through September. Mountain snowmelt in May and June contributes significant runoff, but serious fiooding does not occur unless rainfall accompanies the snowmelt. Peak flooding at Lyons will usually occur within a few hours after a single rainfall event. Flooding is generally of short duration, but may be prolonged signif- icantly by snowmelt. Lyons lies in a natural bowl, on gently sloping land surrounded~by sandstone hills on all sides. The steep stream slopes create swift currents during a flood which produce additional damage. Debris carried by the fast-moving water not only threatens bridges, but may batter houses or other structures on the flood plain. The bridge crossings are often the points of channel restriction, thus raising the water-surface profile. Erosion undercuts and destroys structures which would otherwise receive little damage from inundation. Large quantities of rocks have been deposited in portions of the channel, leaving little capacity for future floods. Early records of floods in the study reach are fragmented and lackinq in detail. Flooding occurred on St. Vrain Creek in 1864, 1876, 1894, 1919, 1941, 1949, 1951, 1957, and 1969. The floods of June 1864 and May 1876 were severe and much valley farmland was flooded. The flood of May 31, 1894, inundated the entire lower part of town. Although 20 homes at Lyons were washed away, no lives were lost. This flood had an estimated peak discharge of 9800 cubic feet per second (cfs) at Lyons, with most of the flow coming from South St. Vrain Creek. , If':! In late July 1919, a series of severe thunderstorms caused flash flooding along St. Vrain Creek. The following is from the Lyons Recorder, dated August 2, 1919: "The heaviest and most destructive cloudburst...in the memory of the oldest inhabitant visited Lyons on Wednesday, July 30, between 2:30 and 3:45 P.M. It took out all the bridges on the North St. Vrain for about 5 miles up and 5 miles downstream. The Longmont and Lyons water mains up the canyon were torn out in many places... miles of road were torn out along the narrow canyon. "The people' living...along the banks of the river were flooded out, and many abandoned their homes for higher ground and safety...homes (in the lower part of town)...were in a roaring sea of water 2 and 3 feet deep." , '\ 4