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Approximately 7 miles north of Craig, the Colorado Game and Fish Commission built Ralph White Dam on Fortification Creek. The storage is 600 acre-feet at the permanent pool elevation. The dam was built for recreational purposes and not flood control. For that reason, the effect of the dam in reducing flood peaks is negligible: therefore, it has little effect on the 100- and 500-year floods. However, the Ralph White Reservoir seems to have had a positive effect on reducing the ice jam problem downstream in the area of Yampa River and Fortification Creek. Cedar Mountain Gulch and Pine Ridge Gulch have been channelized immediately upstream of State Highway 13. pine Ridge Gulch has also been channelized downstream of State Highway 13. These im- provements have reduced the limits of 100-year flooding but have had little effect on the 500-year flooding. 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 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 premium 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 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 county 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 each flooding source studied in detail affecting the county. Peak discharges for floods of the 10-, 50-, 100-, and 500-year recurrence intervals for Yampa River, the lower portion of Fortifi- cation Creek, Brotherton Gulch, Cedar Mountain Gulch, and Pine Ridge Gulch were obtained from previous studies conducted by the 10