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I I I I I I I I I I I I ." I I I I I I .. J .. HYDROLOGY Flooding in the Blue River basin in the vicinity of Breckenridge results from two possible events: high water resulting from snowmelt which occurs in the early summer and thunderstorms of high intensity and small areal distribution which occur during middle and late summer. Flooding from snowmelt is the most prevalent type of flood flow occurring in the Blue River basin. Snowmelt discharge generally begins about April I, and the peak runoff in the basin occurs in mid-June. Snowmelt runoff gradually decreases and the flow of the river recedes in the latter part of June and throughout July and August. Thunderstorms occurring prior to the snowmelt peak runoff generally contribute little to the discharge of the river. This phenomena has been attributed to the drop in temperature immediately pre- ceding a thunderstorm, which reduces the melting of the snow. A second factor is the storage capability of the snowpack. Rain falling on snow that has not reached its threshhold density, that density at which melting begins, can absorb varying amounts of water. The relationship between thunderstorms occurring prior to the date of peak discharge was also investigated by the Denver Water Board through the consulting engineering firm of Tipton & Kalmbach. (1) Floods from thunderstorms occur primarily during July and August. The drop in temperature preceding the thunderstorm does not appre- ciably effect the discharge of the Blue River, as very little snowpack remains in the basin. Losses in the basin are due entirely to infiltration and surface retention; little to no water is stored in the snowpack which has already surpassed its threshhold density. A combination of thunderstorm and snowmelt runoff flood would be a rare event. The flood which occurred on June 17, 1965 was a result of a high intense, but small areal distribution type thunder- storm, occurring at approximately the same time when snowmelt peaked. HYDROLOGIC DATA SOURCES In the process of understanding the hydrology of the Breckenridge area, basic data from precipitation, snow course stations and U.S.G.S. discharge records were examined. The U. S. Weather Bureau maintains a daily recorder for precipitation at Dillon. Three snow course stations maintained by the Soil Conservation Service are located (1) Tipton and Kalmbach, Consulting Engineers, Supplementary Report on Dillon Dam and Reservoir, Board of Water Commissioners, City and County of Denver, Colorado, November, 1958 ~ Leonard Rice Consulting Water Engineers. Inc.