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.~I I I :1 ,Ii 'I I I I I I I I' ,I I ,I I I I 002344 Flood Control Releases at Hoover Dam Step Step 1 Step 2 Step 3 Step 4 Step 5 Step 6 Amount in cubic feet/second o 19,000 28,000 35,000 40,000 73,000 The lowest step, zero cfs, corresponds to times when the regulations do not require flood control releases. Hoover Dam releases are then made on water and power objectives. The second step, 19,000 cis, is based on the Powerplant capacity of Parker Dam. The next step, 28,000 cfs, is the app~oximate maximum release that will not cause damage through the Parker Strip and corre$ponds to the Davis Dam Powerplant capacity which is 28,000 cfs. The fourth step in the COE releases is 35,000 cfs, which corresponds to the Powerp1ant capacity of Hoover D~m in 1987. The present Powerplant capacity at Hoover Dam is 49,000 cfs. At the time Hoover Dam was completed, 40,000 cfs was the approximate maximum nondamagin:g flow downstream from the dam. In addition the "Colorado River Floodway Act" requires that the minimum flood release from Hoover Dam can be no less than 40,000 cfs plus tributary flows. .Prior flood control plans regulated outflow to 40,000 cfs, which forms the fifth step. Releases of 40,000 cfs and greater would result from unusually large floods. The sixth and last step in the series, 73,000 cfs, is the maximum controlled release from Hoover Dam without spillway flow. Runoff forecasts are received from the Colorado River Forecast Service in Salt Lake City, Utah. Flood control releases are required when forecasted inflow exceeds available storage space at Lakes Mead and Powell and allowable space in other upper basin reservoirs. This includes accounting for projected bank storage and evaporation losses at both lakes, plus net withdrawal from Lake' Mead by the Southern Nevada Water Project. The COE regulations set the procedures for :releasing the volume that cannot be impounded. Average monthly rele4ses are determined early in each month and apply only to the current month. The releases are progressively revised in response to updated runoff forecasts and changing reservoir storage levels during each subsequent month throughout the January 1 - July 31 runoff period.~ If the reservoirs are full, drawdown is accomplished to vacate flood control space as required. Unless flood control is necessary, Hoover Dam is operated to meet established down,stream water requirements. Lake Mead end-of-month elevations are driven by downstream demands, Glen Canyon releases, and Mexican: Water Treaty deliveries to Mexico. Lake Mead end-of-month target elevations are not set as are Lake Mohave and Lake Havasu. Normally, Lake Mead elevations rise into March and decline with increasing irrigation deliveries through June or later and then begin to rise again. Lake Mead's storage capacity provides for the majority of 37