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the general V1C1nlty of Estes Park, Glen Haven, and Drake. The estimated peak discharge at the USGS stream gaging station located 6 miles east of Drake was 31,200 cfs, about 4 times larger than the previous recorded peak discharge. ~loods on the Big Thompson River have occurred as follows: June 1921; June 1923; September 1938; June 1949; and, August 1951. 2.4 Flood Protection Measures The sole flood-control structure that will reduce discharges on the stream reaches studied in this report is Ashcroft Draw. At tchis location the Arrowhead Reservoir reduces the 100-year discharge from 1,546 cfs to 654 ds. Levees along the South Platte River help control flooding in some locations, but areas protected from the 100-year flood are not very widespread. '. 3.0 ENGINEERING METHODS ~or the flooding sources studied in detail in the community, standard hydrologic and hydraulic study methods were used to determine the flood hazard data required for this study. Flood events of a magnitude which are expected to be equaled or exceeded once on the average during any 10-, 50-, 100-, or 500-year period (recurrence interval), have been selected as having special significance for floodplain management and for flood insurance premium rates. These events, commonly termed the 10-, 50-, 100-, and SOO-year floods, have a 10, 2, 1, and 0.2 percent chance, respectively, of being equaled or exceeded during any year. Although the recurrence interval represents the long-tcerm average period between floods of a specific magnitude, rare floods could occur at short intervals or even within the same year. The risk of experiencing a rare flood increases when peri'ods greater than 1 year are considered. ~or example, the risk of having a flood which equals or exceeds the 100-year flood (I-percent chance of annual exceedence) in any 50-year period is approximately 40 percent (4 in 10), and for any 90-year period, the risk increases to approximately 60 percent (6 in 10). The analyses reported herein reflect flooding potentials based on conditions existing in the county at the time of completion of this study. Maps and flood eleva- tions will be amended periodically to reflect future changes. 3.1 Hydrologic Analyses .' (10' Hydrologic analyses discharge-frequency recurrence intervals county. were carried out relationships for for each stream to establish the peak floods of the selected studied 1n detail in the The COE provided the 10-, 50-, 100-, and 500-year flood discharges for the South Platte River (Reference 5), and the 10-, 50-, and 100-year discharges for the Cache La Poudre River (Reference 4). The SOO-year discharge for the Cache La Poudre River was obtained from a straight line extrapolation of these values on log- probability paper. The COE studied the diSCharge-drainage area relationship f~r the Cache La Poudre River taking into 9 ,.