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3.0 SOUTH PLA'ITE RIVER BASIN Hydrology data from designated and detailed floodplain reports within the CWCB floodplain information library were obtained and analyzed for the South Platte River Basin. The South Platte River basin was divided into several subregions for the purpose of performing regional regression analyses. A description of each subregion along with the associated regression equation and application criteria are presented below, Figure 1.0, which graphically presents the regional and subregional boundaries for the state, is located in the map pocket at the end of this report. SPL-l: EASTERN PLAINS SUBREGION This subregion includes stream reaches in the plains area of the South Platte River basin. Some of the streams in this subregion originate in the foothills or mountains, The subregion is bounded as follows: . On the south by the South Platte River-Arkansas River basin divide; . On the east by the South Platte River-Republican River basin divide; . On the north by the Colorado-Nebraska and the Colorado-Wyoming state lines; . On the northwest by the Owl Creek-Lone Tree Creek basin divide; . And on the northwest by an arc that passes through the following three locations: I. The confluence of Owl Creek and Lone Tree Creek 2. The town of Bracewell 3. The town of Dent . And on the northwest by the South Platte River mainstem between the town of Dent and the northern boundary of the Urban Drainage and Flood Control District; . On the west by the northern and eastern boundaries of the Urban Drainage and Flood Control District; . On the southwest by the Box Elder Creek-Kiowa Creek basin divide (to the north of Elbert); . And on the southwest by the Kiowa Creek-Comanche Creek basin divide (to the south of Elbert); . And on the southwest by the Kiowa Creek-West Bijou Creek basin divide The regression equation for this subregion is only valid for natural tributary streams that have drainage areas between 2 mi2 and 1090 mil. The mainstems of the South Platte River, Cache La Poudre River, Big Thompson River, and St. Vrain Creek are all exempt from this subregion. A detailed study or other hydrologic analysis must be performed for projects involving those mainstems, and for streams with drainage areas that fall outside of the applicable range. For drainage basins in extreme northeastern Colorado that are affected by sand hills areas, the regression equation may tend to overestimate peakjlow values. 16