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Apr-July Isohyetal maps were constructed for 42 storms. Storms were adjusted to a maximum possible moisture condition for estimating the maximum possible storm. Another adjustment accounted for the rate of moist air inflow. Storms were not transposed from areas outside the basin. The time interval between major storms and types of storms were evaluated in a spaciotemporal manner. The probable maximum snowpack was determined by the synthetic season method. The extent of snow cover was based on that which existed as of the latest occurrence of the first five consecutive days above 32 OF. The April 16th snowpack configuration was adopted as the probable maximum condition. Snow melt runoff factors were established by comparing historic runoff records with the total volume of the snowpack for several subbasins. 2. Spillway Desiqn Flood Studies (Preliminary to Glen Canyon) - Bridge Canyon, Colorado River Storage (Glen Canyon Unit), and Coconino Projects, December 6, 1951, by W. R. Slater, Region 3. Design flood studies were performed to estimate the probable maximum floods for Glen Canyon, Coconino, and Bridge Canyon dam sites. Table 1.5 shows the estimated design floods for each of these dams. Table 1.5.--1951 design floods for Glen Canyon, Coconino, and Bridge Canyon dam sites Site Drainage area (sq. mi.) Peak (ft3/S) Volume (acre-feet) Duration Probable Maximum Rain Floods: Glen Canyon 108,000 417,000 2,063,600 6 days Coconino 26,200 566,000 1,260,100 6 days Bridge Canyon 145,700 552,000 3,002,400 6 days Probable Maximum Snowmelt Floods: Glen Canyon 108,000 196,100 28,460,200 Coconino 26,200 Bridge Canyon 145,700 196,900 26,832,600 Probable Maximum Spring Floods (snowmelt plus rain) : Glen Canyon 108,000 196,100 28,923,900 Coconino 26,200 Bridge Canyon 145,700 196,900 27,304,800 Apr-July Apr-July Apr-July 12