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<br />1941, are caused by short-duration stqrms with peak discharges
<br />occurring in or near the mountains. These flood peaks are attenu-
<br />,
<br />ated by flood plain storage downstream on the plains.
<br />
<br />Major floods have occurred on St. Vrain Creek and its tri-butaries
<br />since 1864. The most significant recorded floods on St. Vrain
<br />Creek occurred in 1921, 1941, 1949, 1951, 1969, and 1973, discharges
<br />of 2,020 cubic feet per second (cfs), '10,500 cfs, 2,970 cfs, 3,920
<br />cfs, 3,060 cfs, and 11,000 cfs, respeqtively, were recorded at the
<br />Lyons gaging station. Major floods of record on Lefthand Creek
<br />had discharges of 1,140 cfs and 785 cfs, at the foothills gaging
<br />station north of Boulder, Colorado, in June 1949 and August 1951,
<br />respectively. A discharge of 812 cfs:was also recorded at State
<br />Highway 287 in September 1938.
<br />
<br />During the week of June 2-7, 1921, a c;ombination of snowmelt runoff
<br />and particularly heavy rains east of Lyons produced flooding condi-
<br />tions in Longmont that were much wors~ than would be expected from
<br />the 2,020-cfs peak discharge recorded!at the Lyons gaging station.
<br />The Boulder Camera reported that st. Vrain Creek had breached its
<br />banks and was 0.75 mile wide at Longmqnt, with floodwater reaching
<br />up to Farmers Mill and the railroad depot. All bridges were swept
<br />away or considered dangerouG. This was regarded as the worst
<br />flood in 25 years.
<br />
<br />On June 22, 1941, a localized cloudburst on South St. Vrain Creek,
<br />upstream of Lyons, caused only minor overbank flooding near Long-
<br />mont due to flood plain storage.
<br />
<br />On June 4, 1949, 16 bridges were rendered useless, irrigation diver-
<br />sion works were damaged, and in Longm6nt several homes and busines-
<br />ses were flooded as a result of prolonged and heavy rain-fall accom-
<br />panied by unusually heavy snowmelt runoff.
<br />
<br />In August 1951, a cloudburst over the!Lyons area again created
<br />severe flooding conditions in Longmont that damaged railroad and
<br />highway bridges and destroyed large ~ounts of harvested grain.
<br />The St. vrain Creek discharge was estimated to be 6,200 cfs near
<br />Interstate Highway 25, at a point 7 miles east of Longmont.
<br />
<br />During the flood of May 9, 1957, on St. Vrain Creek, 3 to 5 inches
<br />of rain fell over the entire basin, c~using a peak discharge of
<br />between 3,060 cfs and 9,450 cfs through Longmont. The resulting
<br />floodwater destroyed irrigation worksi and numerous bridges en-
<br />route to the South Platte River.
<br />
<br />The flood of May 7-8, 1969, on St. Vrain Creek was reported by the
<br />Longmont Daily Times-Call to be the w9rst in 20 years. A combina-
<br />tion of heavy rainfall and snowmelt r~noff produced high flows in
<br />St. Vrain Creek from 2,900 cfs at Lyons to 10,300 cfs at the mouth
<br />near platteville, Colorado. The floo~ing conditions were similar
<br />
<br />6
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