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- 1----- Recorded flood information for the majority of the streams studied in detail within Arapahoe County is nonexistent. Good records do exist for the South Platte River and Cherry Creek. Due to the construction of the Chatfield Dam, the recorded information on the South Platte River is not applicable. As, a result, synthetically derived hydrographs were computed to deter- mine potential flood magnitudes for those streams with relatively small drainage basins in the Denver metropol- itan area. These hydrographs reflect the effects of precipitation characteristic of the basin, ground cover, slope, drainage area, and other physical charac- teristics of the drainage basins. The synthetic hydrograph method was used on Big Dry Creek, Little Dry Creek, Coal Creek, Sand Creek, Piney Creek, Cottonwood Creek, Murphy Creek, Cherry Creek, and the South Platte River. Where available, hydrologic data was compared with other studies completed in the area (References 7, 12, and 13). ~ ,~ <so For the large drainage basins to the east of the Denver metropolitan area, flood magnitudes for the selected frequencies were computed using the U. S. Geological Survey regional analysis outlined in Water Supply Paper 1680 (Reference 14) for Region B, Area 10. The relationship between flood magnitude and frequency, as portrayed in the composite frequency curve in Refer- ence 13, was extrapolated to give a ratio of lOa-year discharge to mean annual discharge as the basis for the regional curve in Figures 4, 5, 6, 7 and 8. The streams whose hydrology was derived from this regional analysis were the upper reaches of Piney Creek and Coal Creek, Lone Tree Creek, Senac Creek, 1-05-4412 Creek, West Box Elder Creek, Box Elder Creek, Kiowa Creek, Wolf Creek, Comanche Creek, Little Comanche Creek, West Bijou Creek, Middle Bijou Creek, and Deer Trail Creek. This curve was used as a comparison for synthetically generated hydrograph flows for each stream in the study. For some streams, the QIOO gener- ated by hydrograph methods is higher than the curve would indicate due to the effects of recent urbanization. Figure 8 provides the discharge data for the South Platte River, while Figures 4, 5, 6, and 7 give the data for the tributary streams. ~ oj The South Platte River peak discharge for the 100- and 500-year frequency flood below the dam was computed to reflect the U. S. Army Corps of Engineers information on the operation of Chatfield Dam. For that reason, the South Platte River does not match the U. S. Geologi- cal Survey regional data. 12