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Mechanical engineer Brent Yamasaki set out amid the recent blistering heat wave to take stock of the giant dams, pumps and pipes that support Diamond Valley Lake in Riverside County, the largest storehouse of water in Southern California.

The reservoir, which he helped build 25 years ago, is 4½ miles long and 2 miles wide and holds back nearly 800,000 acre-feet of water — so much that it would take 20,000 years to fill it with a garden hose.

Stand in a pontoon boat throttling up across its glassy surface, and the reservoir’s jaw-dropping vastness takes hold.

“It’s an inland ocean,” said Yamasaki, regional chief of operations for the Metropolitan Water District, “that Southern California can tap into in the event of a major disaster and in dry times like we’re in right now.”

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