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74 Table 6.3.--Initial reservoir elevations for routing the ULDRS flood through Lake Mead and Lake Powell ULDRS Reservoir st:arting elevation Lake date (feet) Powell May 15 3666.1 June 15 3698.6 July 15 3699.8 August 15 3699.1 Mead May 15 1210.1 June 15 1211. 3 July 15 1219.0 August 15 1217.5 The most critical combination of ULDRS events with the snowmelt flood was determined from reservoir routings. Many possible combinations were tried in order to arrive at the one which produced the highest reservoir elevation. The FHAR computer model was used to compute the flood hydrographs beginning the fifteenth day of May, June, July, and August for each storm combination. The following combinations of storm centerings were evaluated: (1) San Juan followed by Cedar Mountain, (2) San Juan followed by Boulder, (3) Boulder followed by San Juan, and (4) Cedar Mountain followed by San Juan. The Boulder and Cedar Mountain storm centerings were not examined in combination because their close proximity precluded them from occurring within 7 days of each other with magnitudes approaching ULDRS events. The full ULDRS events, which could occur during the August through October time period, were adjusted downward to account for seasonal variations and separation time between storms. Adjustments were made for storms separated by 1, 3, 5, and 7 days. The first storm in the sequence always had the full ULDRS magnitude less the applicable seasonal adjustment, and the second storm was adjusted for both seasonal variation and days between storms. Individual ULDRS events were also centered over the basin to determine the flows at each dam. The numerous combinations of storm centers, storm separation times, and seasonal variations in rainfall magnitudes produced 76 possible flood events for evaluation and comparison. The San Juan storm centering followed by the Boulder storm produced the most critical flood condition at Glen Canyon Dam. Table 6.4 shows the results of this flood analysis. The maximum inflow, outflow, and reservoir elevation are presented for several storm separation intervals and starting dates. The most critical conditions based on the highest reservoir elevations are highlighted and represent the probable maximum floods for the dam. Tables 6.5 through 6.8 list the PMF hydrograph ordinates for each of the floods, and Figures 6.1 through 6.4 show the corresponding hydrographs.