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<br />
<br />As is characteristic of the region, flooding on these
<br />streamis generally occurs between May and September.
<br />Peak arnual flows usually occur in May and June, however,
<br />result~ng 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: downstream 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 />cubic ~eet per second (cfs), 10,500 (cfs), 2,970 (cfs),
<br />3,920 Xcfs), 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 114~ (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 />
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<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 />dischafge 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 of a mile wide
<br />,
<br />at Longmont with flood waters reaching up to Farmers
<br />Mill a~d the railroad depot. All bridges were
<br />swept ~way or considered dangerous. Many claimed this
<br />flood ~o be the worst they had seen in 25 years.
<br />
<br />,;.
<br />
<br />On June 22, 1941, a localized cloudburst on South St.
<br />Vrain Greek, upstream of Lyons, caused only minor over-
<br />bank flooding near Longmont due to flood plain storage.
<br />
<br />On Jun~ 4, 1949, sixteen bridges were rendered useless,
<br />irriga~ion diversion works were damaged, and in Longmont
<br />severa~ homes and businesses were flooded as a result of
<br />prolonged and heavy rainfall accompanied by unusually
<br />heavy snowmelt runoff.
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