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<br />was in 1927. Two bridges on the San Juan River at pagosa Springs <br /> <br /> <br />were lost at a peak river stage of 13.5 feet. McCabe Creek at <br /> <br /> <br />Pagosa Springs left its banks on August 31, 1967 after a cloud- <br /> <br /> <br />burst, and piled mud and debris around several dwellings. The <br /> <br /> <br />Rio Blanco River in Archuleta County has a history of flooding as <br /> <br /> <br />a result of localized heavy rainfall. In 1975, two bridges and <br /> <br /> <br />several ditches along the Rio Blanco River were washed out when the <br /> <br /> <br />river discharge increased from 156 cfs to 2,800 cfs in two hours. <br /> <br />History has shown that major flooding with moderate damage has <br /> <br /> <br />occurred at 50 year intervals while minor flooding and flash floods <br /> <br /> <br />occurred about every 6 years. <br /> <br />FLOOD FLOW FREQUENCY ANALYSIS <br /> <br /> <br />Hydrologic analyses were carried out to establish the peak discharge- <br /> <br /> <br />frequency relationships for floods of the selected recurrence in- <br /> <br /> <br />tervals for each stream in the study area. Since most of the study <br /> <br /> <br />areas are ungauged, a regional relationship relating basin area to <br /> <br /> <br />peak discharge was the principal method used. Gauging stations lo- <br /> <br /> <br />cated within the Animas River Basin and Upper San Juan River Basin <br /> <br /> <br />(Figure 8) were used to establish the regional relationship for each <br /> <br /> <br />basin. The method developed by the united States Water Resources <br />Council (Reference 12) was used to determine the flood flow frequency <br />relationship based on log-Pearson Type III Distribution for each <br />selected gauging station. Gauqing stations used for developing the <br />regional relationship were selected according to the criteria of <br />(1) stations which have 10 or more years of record and (2) stations <br /> <br />-11- <br />