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The major portion of the Dry Creek No. 1 Basin is located outside the limits of the study area. Runoff fram this area contributing to peak discharges within the study reach is limited by the Colorado and Southern Railroad, which diverts most of the upstream runoff north to St. Vrain Creek. Rainfall data for the synthetic hydrologic analysis were taken fram a rainfall/runoff information report (Reference 41) . The discharges computed using the HBC-l program were verified using the Plains Region equations developed by the USGS (Reference 32). Peak discharges were also verified by the BeB runoff prediction method (Reference 42). Discharges for the SOO-year floods of all streams were checked by straight-line extrapolation of frequencies previously determined using the procedure of the USGS (References 9 and 10), and compared to the USACB Standard Project Flood data when available. The hydrologic analyses for this study were revised to include information presented in floodplain info~tion reports for Lefthand Creek, Dry Creek No.1, and St. Vrain Creek (References 1 through 4). The Lyons streamflow gage, located on the left bank of St. Vrain Creek 0.4 mile downstream from the confluence of North St. vrain Creek and South St. Vrain Creek, has been in operation since 1895. The flows recorded are partly regulated by small diversions above the gage station. Significant peak flood discharges and stages recorded during this period are presented in Table 2 (Reference 43). Table 2. Historic Flood Peak Discharges and Stages at Lyons Gage, St. Vrain Creek Max~ Discharge (Cubic Feet per Second) Date Stage (Feet) July 30, 1919 June 22, 1941 August 3, 1951 7.90 . 8.06 5.37 9,400 10,500 3,920 This report is based upon data generated for the June 1972 and September 1972 studies of Lower and upper St. Vrain Creek by the USACB (References 8 and 44, respectively). The diSCharge-frequency relationships in the St. Vrain Creek basin at the Town of Lyons were based on a statistical analysis of the stream gaging records of the St. Vrain Creek at Lyons. Synthetic unit hydrographs were developed for the St. vrain Creek basin and its subdrainage basins of North St. Vrain Creek and South St. Vrain Creek to help define the flow characteristics within the basin. The hydrographs were used for stream routing through Button Rock Dam to Lyons, and downstream from Lyons to determine the discharges throughout the length of the stream. 35