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to those of the June 1921 flood, with flood plain width averaging 1.0 mile. Extensive damage was done to bridges, with severe ero- sion nearby to roads and along the channel banks. Public and pri- vate property damage amounted to $50,000. Lefthand Creek also produced a large flood on May 7-8, 1969, with the primary damage being done to the South Pratt Parkway bridge, which was ultimately destroyed by the floodwater. There is little known regarding floods of record other than what was stated con- cerning the gaged discharges. There are no existing gage data for the floods on Lefthand Creek later than May 1957. The largest flood on record was the one that occurred in June 1949. . Flood problems in the area have been the result of not only rare storm events, but also improper flood plain development. Visual accounts of floods have noted the debris collected by the flood- water, including natural debris such as trees, rocks, and soil, but consisting chiefly of items foreign to the flood plain, such as houses, bridges, automobiles, heavy equipment, lumber, house trailers, and butane storage tanks. with these items obstructing bridges and culverts, flood levels rise and cause more extensive damage. property that was not structurally damaged by flood depths and velocities experienced much damage and cleanup expense result- ing from mud, silt deposition, and erosion. Scenes of past flooding in Longmont are shown in Figures 2-9. 2.4 Flood Protection Measures There is little evidence of flood protection measures along the study segment of St. vrain Creek, except the channelization be- tween Sunset Street and the Colorado and Southern Railroad Bridge. There is a line of low levees along the trailer park on the south side of this segment. There are three dams in the upper mountain segments of St. vrain Creek. Button Rock Dam was completed on North St. Vrain Creek in 1969, 6 miles west of Lyons, to store the municipal water supply. It was not designed to store floodwater; but, during the May 1969 flood, it reduced the magnitude of the discharge that could have inundated Longmont. Both the Longmont Dam, located 1 mile downstream of the Button Rock Dam, and Beaver Reservoir, near the South St. Vrain Creek headwaters, have no effect on the flood discharge in Longmont. . Lefthand Creek has been channelized along the segment from pike Road (State Highway 22) to the mouth. The channel from Pike Road to South Pratt Parkway is grass lined and is designed to contain a flow of approximately 3,650 cfs. From South pratt Parkway to its mouth, Lefthand Creek has been channelized; however, the overbanks remain unimproved. 7