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apply to all areas containing or suspected of containing: (a) natural hazards which include, but shall not be limited to flooding; concentrated runoff; inadequate drainage;... Section 4.2.3(e) of the Subdivision Regulations requires that any proposed subdivision that lies in an identified flood hazard area or, in the absence of officially identified areas, that is suspected by the County Planning Commission to lie in a flood hazard-prone area, the subdivider shall also submit a study by a professional engineer for approval. A detailed requirement for improvements in areas of potential flood hazard is included in the Archuleta County Building Permit regulations. ~ 3.0 ENGINEERING METHODS ~ For flooding sources studi~d in detail in the town and county, standard hydrologic and hydraulic study methods were used to determine the flood hazard data required for this study. Floods having recurrence intervals of 10, 50, 100, and 500 years have been selected as having special sig- nificance for flood plain management and for flood insurance premium rates, The analyses reported here reflect current conditions in the watersheds of the flooding sources. 3.1 Hydrologic Analyses Hydrologic analyses were carried out to establish the peak discharge- frequency relationships for floods of the selected recurrence intervals for each stream studied in detail in the town and county. . A regional relationship of basin area to peak discharge was used for the Rio Blanco. Gaging stations located within the upper San Juan River basin (Table 1) were used to establish the regional relationship. The method developed by the U.S. Water Resources Council (Reference 2) was used to determine the floodflow-frequency relationship, based on a log-Pearson Type III distribution for each selected gaging station. Gaging stations used for developing the regional relationship were selected according to the fOllowing criteria: (1) stations having 10 or more years of record, and (2) sta- tions where peak flows are not affected by regulation and diversion, . Because the annual peak discharges published in U.S, Geological Survey Water-Supply Paper Nos. 1683, 1925, and 2125 (References 3, 4, and 5, respectively) and Water Resources Data for Colorado (Refer- ence 6) were a mixture of flood flows caused by snowmelt and rainfall events, the regional relationship established by using these annual peak discharges is not accurate for intense rainfall-produced floods. A more detailed method was used for each gaging station to develop regional relationships for this area. The procedures are described as follows: (1) separating the rain and snowmelt data at each gaging station; (2) developing statistics for each type of event; (3) statistically combining the two frequency curves (streamflow records were obtained from the U.S. Geological Survey, Denver Federal 7