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1.6 Terrain Review The region between the Continental Divide and the 103rd meridian is topograph- ically one of the most complex regions in the conterminous United States. It is a region of steep slopes, narrow enclosed valleys, and open plains. To gain a greater understanding of this complex reg-ion, several of the study participants undertook an aerial reconnaissance of the entire region. Of particular importance was the topography at the locations of some of the more significant rainstorms that have occurred within the region: Gibson Dam, Warrick and Springbrook, MT; Savage ton, WY; Big Thompson, Cherry Creek, Plum Creek, and Penrose, CO; and McColleum Ranch, NM. This aerial survey took place on three separate flights, and was conducted approximately 2,000-4,000 ft above the terrain. Figure 1.2 shows a schematic of the flight paths. A photographic record was made during these overflights. These photographs were referred to during early stages of the study to aid in understanding relative terrain influence. 1.7 Previous PMP Estimates for the CD-I03 Region The PMP values for this study are termed generalized or comprehensive estimates. By this it is meant isolines of PMP are g-iven on index maps and depth-area relations are provided allowing determination of average storm- centered PMP for any drainage within the region. The present study has combined the latest storm data and current knowledge of the precipitation process to develop these estimates of PMP. Results from Weather Bureau Teclmical Paper No. 38 (U.S. Weather Bureau 1960), for the region between the Continental Divide and the 105th meridian, and HMR No. 51, for the reg-ion between the 105th and 103rd meridians, have been superseded by the present study. Through the years, the Hydrometeorological Branch has provided PMP estimates for particular basins often referred to as individual drainage estimates. These estimates were provided if generalized PMP studies were not available, or if available generalized PMP estimates did not provide estimates for area sizes as large as tbe drainage under investigation. Of the more recent individual studies in the region considered in this report, only the one for the South Platte River, Hydrometeorological Report No. 44, "Probable Maximum Precipitation over South Platte River, Colorado, and Minnesota River, Minnesota (Riedel et al. 1969) has been published. In some situations, because of basin shape, unusual orographic considerations, areal or spatial distribution developed for the individual basin specific estimate, or other factors, the individual drainage estimate may take precedence. However, the applicability of the individual drainage estimate must be carefully evaluated on a case-by-case basis by a qualified hydrometeorologist, as the need arises. 1.8 Application of HMR No. 52 to PMP frOlll this Study Hydrometeorological Report No. 52, "Application of Probahle Maximum Precipitation Estimates - United States East of the 105th Meridian" (HMR No. 52) (Hansen et a1. 1982), was completed as an aid .... .in adapUng- or applying PMP estimates from HMR No. 51 to a specific drainage." The procedures in HMR No. 52 are intended for application to nonorog-raphic generalized PMP estimates and were done essentially independent of the base level PMP analyses. The present CD-l03 study has introduced new delineations that limit the extent of nonorographic PMP within the 1030-1050 region. This delineation is represented by the orog-raphic 7