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<br />Flood of 2-7 June 1921. Heavy rains combined with snowmelt runoff caused fiooding in the l,ongmont area
<br />early in June of 1921, A total of 5.87 inches of rain was recorded at Longmont from 2 to 7 June, The Boulder
<br />Camera of 7 June 1921 reported that St Vrain Creek at Longmont was "way out of its banks" and "three-
<br />quarters of a mile wide, Water is up to the Farmer's Mill and the depot All bridges in the city are out or
<br />dangerous," The peak discharge at l.yons was 2,020 cis on 7 June which was not indicative of conditions at
<br />Longmont because of heavy rains downstream of Lyons,
<br />
<br />Flood of Seotember 1938. The flood of September 1939 was confined mostly to rural areas downstream
<br />of l.ongmont
<br />
<br />Flood of 22 June 1941. The maximum peak discharge on St Vrain Creek at l.yons is 10,500 cis on 22 June
<br />1941, Little information is readily available on this flood except that it originated mostly on South St Vrain Creek
<br />and that it peaked rapidly at Lyons." It is assumed that an extremely localized cloudburst occurred over South
<br />St Vrain Creek a short distance upstream from Lyons and that minor bank overflows occurred in rural areas
<br />upstream from Longmont
<br />
<br />Flood of 4 June 1949, St Vrain Creek peaked at 2,970 cfs at Lyons on 4 June 1949, The creek was out
<br />of its banks most of the month of June due to heavy and prolonged rainfall and unusually heavy snowmelt
<br />runoff. Sixteen bridges were damaged and out of use, with two completely destroyed, Irrigation headworks
<br />were extensively damaged. In Longmont, 10 homes and 5 businesses were flooded,
<br />
<br />Flood of 3 August 1951, Heavy rain occurred in the St Vrain Creek basin during the afternoon and night of
<br />3 August 1951, Lyons received 6,3 inches from a cloudburst that began about 6 p,m, Flooding resulted in the
<br />reach from Lyons to the mouth of St Vrain Creek. Flood duration was less than 12 hours, The peak discharge
<br />at Lyons was 3,700 cis and at a point 7 miles of l.ongmont was estimated at 6,200 cfs, Railroad and highway
<br />bridges near Longmont were severely damaged, Farmers in the St Vrain valley lost much grain already
<br />shocked in the fields,
<br />
<br />Flood of 9 May 1957, During the night of 8 and 9 May, approximately 3 to 5 inches of rain fell over the entire
<br />basin of St Vrain Creek, The rain began about 10 p,m, and stopped about 6 a,m, with St Vrain Creek peaking
<br />at Lyons about 1 a,m. on 9 May at 3,060 cfs, A peak of 9,40;0 cfs reached the Mouth of the creek at 12:15 a,m,
<br />on 10 May. Irrigation diversion works and bridges between Lyons and l.ongmont were damaged or destroyed,
<br />
<br />Flood of Mav and June 1969, Heavy rainfall combined with snowmelt runoff caused prolon!)ed hi!)h flows
<br />on St Vrain Creek, The worst flooding occurred on 7 and 8 May and from 15 to 21 June, Roads and bridges
<br />along the stream were extensively damaged, stream banks were eroded, farmlands were flooded, The peak
<br />dischar!)e at Lyons was 2,900 cis on 7 May and at the creek mouth, 10,300 cis on 8 May,
<br />
<br />4.1.12 BIG AND LITTLE THOMPSON RIVER BASIN
<br />
<br />Flood History, Little information is available relative to historic flooding on the Little Thompson River
<br />probably because of its rural nature which has resulted in little flood damage during flood events. Some flood
<br />
<br />Colorado Flood
<br />Hydrology Manual
<br />
<br />4,14
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<br />DRAFT
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