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1 1 1 1 - 8 - TABLE 2 DRAINAGE AREAS 1 1 1 Blue River above gage Blue River above Tarn Lehman and Carter Gulch Illinois Gulch Sawmill Gulch French Gulch North Barton Gulch Swan River 119.0 mi2 43.5 3.0 3.5 2.5 11.0 4.6 38.5 1 Most of the drainage basins in the area are remarkably similar. All the basins have their headwaters either on the Continental Divide or on a major drainage divide. The slopes are extremely steep, ranging from 8 to 13 percent. 1 Several lakes or man-made structures were found to exist in the basin above the town of Breckenridge. These structures were evaluated as to their ability to act as flood retention or de- tention structures. The lakes that occur in the subbasins on the tributaries to the Blue River were found to be small enough to be considered negligible for flood peak reduction capabilities. This would particularly be true during the time of a thunderstorm in which a very large volume of runoff would be passing through the structure in a very short time period. A major structure on ~he Blue River above Dillon, the Goose Pasture Tarn, was found to be the only significant reservoir in the basin that would affect flood flows. As described in a subsequent section the reservoir plays an important role in reducing flood peaks due to runoff from thunderstorms but the reduction of flood peaks due to snowmelt would be negligible. 1 1 1 I. 1 I DESIGN PRECIPITATION AND STORMS 1 Rainfall intensities given in Technical paper 40 were used to synthetically develop flood flows. As previously mentioned, the '- most likely rainfall flood producing event is a thunderstorm of high intensity and small areal distribution occurring in July and August. Data presented in the reports prepared by Mr. Mulvey concerning the meteorology of the upper basin were used to define the area extent and probable location of maximum precipitation in the basin. Rainfall intensities from Technical Paper 40 were 1 1 1 ~ Leonard Rice Consulting Water Engineers. Inc.