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,"' .. , , o Another crest on the following day flooded houses again in the lower areas of town and washed out 300 yards of railroad track east of Lyons. The peak discharge on July 30 was later computed to be 9400 cfs. The right bank of North St. Vrain Creek was flooded to a width of 300 feet (Reference 2). The largest peak discharge of record on St. Vrain Creek at Lyons was 10,500 cfs on June 22, 1941. This flood originated mostly on South St. Vrain Creek, and the creek peaked very rapidly with floodwaters receding quickly. It is assumed that an extremely localized cloud- burst occurred over South St. Vrain Creek a short distance upstream from Lyons. The effects of the June 4, 1949 flood were felt largely downstream of Lyons. Prolonged rainfall and heavy snowmelt kept St. Vrain Creek out of its banks in rural areas during most of the mOnth of June. Bridges, roads, and irrigation headworks were damaged. Lyons received 6.3 inches of rain from a cloudburst-type storm that began about 6 p.m. on August 3, 1951. This combined with generally heavy rains over the basin to cause flooding from Lyons to the mouth of St. Vrain Creek. The flood lasted for less than 12 hours. Severe damage resulted to Colorado Highway 7 along South St. Vrain Creek. In the rural areas downstream from Lyons, many grain shocks were washed from the fields. On May 8 and 9, 1957, approximately 3 to 5 inches of rain fell over the entire basin of St. Vrain Creek. The rain began at approximately 10 p.m. and stopped at approximately 6 a.m. On May 9, at approximately 1 a.m., St. Vrain Creek peaked at Lyons at 3060 cfs. The flood damaged and destroyed irrigation diversion works and bridges downstream from Lyons. In 1969, heavy rainfall combined with snowmelt caused prolonged high flows on St. Vrain Creek. The worst flooding occurred on May 7 and 8 and from June 15 to June 21. Roads and bridges along the stream were extensively damaged, streambanks were eroded, and farmlands were flooded. On May 7, the peak discharge at Lyons was 2900 cfs. Photographs of the 1969 flood are shown on Figures 2, 3, 4, and 5. 2.4 Flood Protection Measures The channels of North St. Vrain, South St. Vrain, and St. Vrain Creeks and their tributaries in the study area are essentially in their natural state. There are no flood control structures in Lyons. Button Rock Dam is located 6 miles west of Lyons on North St. Vrain Creek. This dam was built for the'Longmont municipal water supply and normally has little flood storage capacity. Longmont Dam, 1 mile downstream from Button Rock Dam, and Beaver Reservoir, near the headwaters of South St. Vrain Creek, are considered to have little effect on peak discharges in the study reach. 5