The Olivenhain Dam and  Reservoir is located within the Elfin Forest Recreational Reserve and serves as an emergency water storage facility for San Diego County, to be used in the event of an earthquake or other disaster severing incoming water transportation lines.  It was the first roller-compacted concrete dam built in California and was designed to remain fully functional during a magnitude 7.5 earthquake.  Completed in 2003, the reservoir can hold 24,000 acre-feet of water.

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