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PRECIPITATION The principle cause of flood flows in the Blue River Basin is runoff from snowmelt. According to stream gage records, about 97% of the annual peak flows in the Bl.ue River Basin have been the result of runoff from snowmelt (Ref. 2). This runoff usually starts about the first of April, peaks by mid-June and then recedes back to a normal flow by mid-July or August. Rainfall does occur over the basin during these months, but usually has a minor effect on the runoff during the period before the peak. This is thought to be the result of a lowering of temperatures which frequently accompanies a rainfall storm in the mountains, thus retarding the snowmelt runoff. lvarm rains on a snowpack seem to have little effect on snowmelt unless accompanied by warmer temperatures (Ref. 2). After the snowmelt peak, rainfall usually increases the runoff. However, these late Sllimner rain storms rarely cause flood flows, as indicated by gage records in the area. An analysis (Ref. 3) of the gage records within Eagle county showed that up to the SOO-year frequency events, the snowmelt events were found to produce higher discharges than the rainfall events for all but one gage analyze1. Therefore, in this report, the hydrology analysis was performed based upon snowmelt events, except for small drainage basins. RUNOFF COMPUTATIONS A Log Pearson Type III Analysis and a regression analysis of the data, utilizing gage records on all streams not effected by Dillon Reservoir, were used to determine the 10-, 50- 100-, and SOO-year peak flood flows on streams studied in detail in Summit County (Ref. 4 and 5). The only exception is the Blue River below Dillon Reservoir, where flood flows are controlled by Dillon Reservoir and cannot be analyzed by this method. The hydrology for this area will be discussed in another report. -5-