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ace 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 flood plain management and for flood insurance rates. These events, commonly termed the 10-, 50-, 100-, and 500-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-term average period between floods of a specific magnitude, rare floods could occur at short inter- vals Or even within the same year. The risk of experiencing a rare flood increases when periods greater than 1 year are considered. For example, the risk of having a flood which equals Or exceeds the lOO-year flood (1 percent chance of annual exceedence) in any 50-year period is approxi- mately 40 percent (4 in 10), and, for any 90-year period, the risk in- creases to approximately 60 percent (6 in 10). The analyses reported herein reflect flooding potentials based on conditions existing in the community 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 Hydrologic analyses were carried out to establish the peak discharge- frequency relationships for each flooding source studie4 in detail affecting the community. A streamflow gaging station on the Big Thompson River near Lake Estes and another on Fish Creek near Lake Estes were the principal sources of data for defining discharge-frequency relationships for the streams. These two gages have operated continuously from 1947 through 1977. A third gaging station was located on Fall River, but was only in operation for 9 years, between 1945 and 1953. Values of the 10-, 50-, 100-, and 500-year peak discharges were obtained from a log-Pearson Type III distribution of annual peak flow data for each of these streams (Reference 4). These frequency discharges for Black Canyon Creek and the Big Thompson River above its confluence with the Fall River were obtained using the USGS regional relationships (Reference 5). The discharges developed from these relationships were consistent with gaging station records downstream. Discharges on Dry Gulch were determined using the Storm Water Manage- ment Model (SWMM) (Reference 6). Peak discharge-drainage area relationships for the Big Thompson River, Fall River, Fish Creek, Black Canyon Creek, and Dry Gulch are shown in Table 1. A summary report of procedures used to determine frequency discharges for this study was prepared upon completion of the Estes Park hydro- logy. This report was submitted to FEMA, the CWCB, Larimer County, and the Town of Estes Park for review and comment in March 1977 (Reference 7). 8