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. As is characteristic of the region, flooding on these streams generally occurs between May and September. Peak annual flows usually occur in May and June, however, resulting from a combination of snOWMelt runoff and spring rains. Floods having the most damaging effect in the Longmont area, such as the flood of June 2-7, 1921, occur when a long-duration rainstorm forms over the St. Vrain Creek basin with the heaviest rainfall accumulation downstream of the Lyons gaging station. Other floods, such as that of June 22, to those of the June 1921 flood, with floodplain width averaging 1.0 mile. Extensive damage was done to bridges, with severe erosion nearby to roads and along the channel banks. Public and private 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 concerning 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 floodplain development. Visual accounts of floods have noted the debris collected by the floodwater, including natural debris such as trees, rocks, and soil, but consisting chiefly of items foreign to the floodplain, 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 veloci ties experienced much damage and cleanup expense resulting from mud, silt deposition, and erosion. Scenes of past flooding in Longmont are shown in Figures 9 through 16. The Town of Lyons lies in a natural bowl, on gently sloping land surrounded by sandstone hills on all sides. The steep stream slopes create swift currents during a flood, which produce additional damage. Debris carried by the fast-moving water not only threatens bridges, but may batter houses or other structures on the floodplain. The bridge crossings are often the points of channel restriction, thus raising the water-surface profile. Erosion undercuts and destroys structures which would otherwise receive little damage from inundation. Large quantities of rocks have been deposited in portions of the channel, leaving little capacity for future floods. . Barly records of floods in the study reach are lacking in detai 1. Flooding occurred on St. Vrain 1876, 1894, 1919, 1941, 1949, 1951, 1957, and 1969. fragmented and Creek in 1864, The floods of . 21