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<br />As is characteristic of the region, flooding on these <br />streams generally occurs between May and September. <br />Peak ann~al flows usually occur in May and June, however, <br />resulting from a combination of snowmelt runoff and <br />spring rains. Floods having the most damaging effect <br />on Longmont area, such as the flood of June 2-7, 1921, <br />occur when a long duration rain storm forms over the <br />St. Vrain Creek basin with the heaviest rainfall accumu- <br />lating downstrean of the Lyons gaging station. Other <br />floods, such as that of June 22, 1941, are caused by <br />short duration storms with peak discharges occurring in <br />or near the mountains. These flood peaks are attenuated <br />by flood plain storage downstream on the plains. <br /> <br />Major floods have occurred on the St. Vrain Creek and its <br />tributaries since 1864. The most significant recorded <br />floods on the St. Vrain Creek occurred in 1921, 1941, <br />1949, 1951, 1969, and 1973, when discharges of 2,020 <br />c~bic feet per second (cfs), 10,500 (cfs), 2,970 (cfs), <br />3,920 (cfs), 3,060 (cfs), and 11,000 (cfs), respectively, <br />were recorded at the Lyons, Colorado gaging station. <br />Major floods of record on Left Hand Creek had discharges <br />of 1140 (cfs), and 785 (cfs), at the foothills gaging <br />station north of Boulder, Colorado in June of 1949, and <br />August of 1951, respectively. A discharge of 812 (cfs) <br />was also recorded at State Highway 287 in September <br />1938. <br /> <br />In the week of June 2-7, 1921, a combination of snowmelt <br />runoff and particularly heavy rains east of Lyons pro- <br />duced flooding conditions in Longmont that were much <br />worse than one would expect from the 2020 (cfs) peak <br />discharge recorded by the Lyons gaging station. The <br />Boulder Camera reported that the St. Vrain Creek had <br />breached its banks and was three quarters af a mile wide <br />at Longmont with flood waters reaching up to Farmers <br />Mill and the railroad depot. All bridges were <br />swept away or considered dangerous. Many claimed this <br />flood to be the worst they had seen in 25 years. <br /> <br />On June 22, 1941, a localized cloudburst on South St. <br />Vrain Creek, upstream of Lyons, caused only minor over- <br />bank flooding near Longmont due to flood plain storage. <br /> <br />On June 4, 1949, sixteen bridges were rendered useless, <br />irrigation diversion works were damaged, and in Longmont <br />several homes and businesses were flooded as a result of <br />prolonged and heavy rainfall accompanied by unusually <br />heavy snowmelt runoff. <br /> <br />7 <br />