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characteristically have high peak discharges of short duration, lasting only a few hours. As the flood flow passes through the upper portions of the basin, little channel storage exists to attenuate or reduce the flood discharge. As the flood moves into the flatter areas below the canyon mouth, channel and flood plain storage slow the flow velocities and reduce the flood discharge and flood elevation. and study findings. State-of-the-art hydrology study methods were used in completing the hydrologic analysis. The study. methods considered included: The confined and hydraulically steep channel characteristic of the Big Thompson River above the canyon mouth, will sustain hazardous velocities throughout the canyon bottom for major flood events. Properties not subject to flooding may be subject to significant hazard and damage through the erosion of the channel sides by the high flOOd velocities. , , I , , , I I , I , I I I I , 1. Log-Pearson Type III Frequency Distribution, Water Resources Council Bulletin No. 17 (Ref. 5) 2. "Manual for Estimating Flood Characteristics of Natural-Flow Streams in Coloradow, Technical Manual No.1, V. S. Geological Survey and Colorado Water Conservation Boarq (Ref. 6) 3. Storm Water Management Model, Environmental Protection Agency (Ref. 7) Hydrologic AnalySis The Log-Pearson Type III frequency distribution served as the primary basis for estLmating the flood discharge-frequency. The other two methods served as a means of checking the study results. The Log-Pearson Type III method relies on the historic stream flow records at various gages on the river. The stream gages on the 8ig Thompson River considered in the study are listed below: A hydrologic study was completed to estimate the peak discharges for the 10-, 50-, 100- and SOO-year recurrence interval floods on the Big Thompson River and tributaries within the study area. The results of this study serve as a basis for determining the flood profiles for the various frequency floods and ultimately, the basis for flood plain delineation, flood plain insurance and flood plain management. Period of Years of Record Record Gaqe Name Big Thompson River, 1888-1976 49 at mouth of Canyon Big Thompson River, 1918-1955 38 below Powerhouse North Fork Big Thompson River, 1947-1976 30 " Drake Big Thompson River, 1947-1976 30 near Estes Park Big Thompson River, 1930-1976 '" ., Estes Park The hydrology study was completed with the assistance and cooperation of various federal, state and local agencies mentioned earlier. Two review meetings were held with the various agencies mentioned previously tc discuss the hydrology study methods -10- -11-