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shows that the San Juan River basin is rarely subject to major flooding because the heavy snowcover runoff from the San Juan Moun- tains keeps the rivers scoured to the required capacitYe New communi- ties, such as the Upper Vallecito Resort Development, are subject to cloudburst type floods. Short-duration, heavy rainfall on steep- gradient mountain streams causes very destructive flooding because of the high-velocity flows. The flood of September 1970 at Upper Vallecito Creek was caused by , an extremely heavy rainfall of 4.5 inches in a 6- to 12-hour period. Many vehicles, homes, and trailers were damaged or destroyed. The total cost of the flood loss exceeded $60,000. Major flooding . occurred throughout the Animas, Florida, and Los Pinos Valleys in October 1911 and July 1927. The October 1911 flood was the worst flood on record for the San Juan River drainage basin. 2.4 Flood Protection Measures No specific flood protection works have been undertaken in the study area. Although the Vallecito Reservoir can regulate flow along Los Pinos River, it was designed for irrigation purposes. It does, however, provide some protection from minor flooding. However, some flood protection measures are provided in La Plata County Flood Hazard Regulations for areas within the 100-year flood plains. Flood Hazard Regulations have been established in the county that restrict certain types of development along the streams in the county. 3,0 ENGINEERING METHODS . For the flooding sources studied in detail in the county, 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 ~ 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 10, 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 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 here reflect flooding potentials based on conditions existing in the county at the time of completion of this study. Maps and flood elevations will be amended periodically to reflect future changes. < 7