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2.4 Flood Protection Measures No flood protection measures affecting peak floodflows or flood elevations on San Miguel River are in existence at Naturita. Numerous storage reservoirs, designed for water supply, recrea- tion, and irrigation, as well as numerous irrigation diversions, do affect natural flow on San Miguel River. Because these struc- tures were not designed for flood control purposes, they offer no flood protection. Some of the larger storage reservoirs include Gurley, Trout Lake, Hope Lake, Lone, Lillyland, and Miramonte. Tributary 1 has been channelized into an 8-foot by 4-foot rectan- gular channel with concrete sides and earth bottom. The channel contains the lOa-year base flood. There is no flood plain management in the Town of Naturita. 3.0 ENGINEERING METHODS For the flooding source 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 equalled or exceeded once on the average during any 10-, 50-, 100-, or SOO-year period (recurrence interval) have been selected as having special significance for flood plain management and for flood insurance premium rates. These events, commonly termed the 10-, 50-, 100-, and SOO-year floods, have a la, 2, 1, and 0.2 percent chance, respectively, of being equalled 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 occurrence) 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 here 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 floods of the selected recurrence intervals for the stream studied in detail in the community. For San Miguel River at Naturita, peak discharges for the 10-, 50-, 100-, and SOO-year floods were determined using a regional floodflow-frequencyanalysis (Reference 6). For the regional study, U.S. Geological Survey stream gage records were analyzed 6