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Aspen has a long history of floods, the earliest record occurring in 1880 (Reference 3). A flood that occurred in June and July 1917 is considered the most severe in the Aspen area with a peak of 3,170 cubic feet per second (cfs) for the Roaring Fork River. In addition, records show that four flood peaks over 2,000 cfs occurred from 1915 to 1921. Since 1921, no flows of this magnitude have been recorded. The largest recorded flood on Castle Creek occurred in 1918 with a peak flow of' 1,090 .cfs. For Maroon Creek, a peak of 836 cfs was recorded in 1980 (Reference 4). , 2.4 Flood Protection Measures . There are several small reservoirs in the Roaring Fork watershed above Aspen and the Twin Lakes diversion for the purposes of irri- gation and water supply. The effect of these is insignificant during a flood event. A 500-foot reach of the Roaring Creek River, located approximately 800 feet upstream of North Mill Street was widened in December 1984. In recent years, Aspen has attempted to control the potential for flood damage by trying to limit construction in flood-prone areas. No other form of flood plain management is practiced by Aspen. ~.O ENGINEERING METHODS For 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 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 intervals 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 100-year flood (1 percent chance of annual exceedence) in any 50-year period is approximate~y 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 community at the time of completion of this study. Maps and flood elevations will be amended periodically to reflect future changes. . . 4