Seven Oaks Dam - History

History

In the wake of severe flooding that killed between 90–100 people in Southern California during early 1969, the USACE began an ambitious effort to improve levees and dams on the Santa Ana River system known as the Santa Ana River Mainstem Project. Among the works proposed under the project were an expansion of the massive Prado Dam, which despite filling to capacity did not fully prevent heavy flooding damages in Orange County, and the construction of the $530 million "Mentone Dam" across the Santa Ana north of Mentone in San Bernardino County. However, the Mentone Dam site was controversial because it would affect groundwater recharge, was aesthetically unpleasing and lay on unstable sediment deposits directly above the San Andreas Fault. In response to public opposition, the USACE replaced the Mentone proposal with the $304 million Upper Santa Ana River Dam, later to be called Seven Oaks Dam, in a bedrock canyon one mile (1.6 km) upstream in the southern San Bernardino Mountains.

The contract for Seven Oaks Dam was awarded in 1994 and construction work began in May. A system of conveyor belts totaling 3.5 miles (5.6 km), with a capacity of 3,400 short tons (3,080 t) per hour, was built to transport materials to the dam site. Fill used in the embankment was excavated from the Santa Ana River canyon directly below the dam, the alluvial fan of the river north of Mentone, and a cut in a ridge directly southeast of the dam that now serves as the dam's spillway. The dam was built in ten alternating zones of dirt, rock and clay that ultimately totaled 38,000,000 cubic yards (29,000,000 m3). When completed and dedicated in January 2000, the Seven Oaks Dam was the 12th highest dam in the world and was one of the last major dams built in the United States. At $450 million, the final cost of the dam significantly exceeded the USACE's projected cost. The federal government shouldered 70.47% of the total cost, while Orange County paid 27.09%, San Bernardino County 1.71%, and Riverside County 0.73%.