Association of State Dam Safety Officials Marks 50th Anniversary of Lower Hell Hole Dam Failure

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Anniversary highlights need for emergency planning during all phases of construction and operation

On December 23, 1964, after five days of heavy rainfall, the partially completed Lower Hell Hole Dam failed, causing 30,000 acre-feet of water and 700,000 cubic yards of rock to surge 60 miles down the Rubicon River canyon before terminating into Folsom Lake four hours later.

The Lower Hell Hole Dam, located about 70 miles east of Sacramento, California, was designed as a 410-foot high zoned rockfill structure. During the first year of construction, progress was slower than anticipated, and the dam was only completed to a height of 220 feet before the start of the flood season. In late December 1964, record rainfall of 22 inches over five days filled the reservoir, causing the incomplete dam to fail.

Thankfully, no one was killed as a result of the failure, but rock from the failed dam was carried downstream for miles and five bridges, including two suspension bridges and the California Highway 49 bridge over the American River, were washed out before the water flowed into the Folsom Reservoir. The lack of more severe consequences can be attributed to the remote location of the reservoir as well as the event occurring during a time of the year when downstream recreational use was low.

Dam safety experts attributed the cause of failure to incomplete construction of the dam coupled with record rainfall. The dam design was not judged to be a contributing factor, and it was subsequently reconstructed in the wake of lawsuits for damages. Lower Hell Hole Dam is owned and operated by the Placer County Water Agency and continues to provide flood control, water storage, power generation, and recreational opportunities.

The anniversary of this dam failure is a reminder that, as more people live and work in locations downstream from new and existing dams, it is important to be prepared and to have complete and routinely practiced emergency action plans.

“Emergency action plans are valuable tools that can help save lives by putting important safety and evacuation procedures in place before an emergency occurs,” said Lori Spragens, executive director of the Association of State Dam Safety Officials (ASDSO). “Everyone has a role to play in creating a future where all dams are safe, and the anniversary of the Lower Hell Hole Dam failure reminds us of the importance of understanding the risks associated with potential dam incidents and failures.”

According to the California Division of Safety of Dams, California has 1,250 state-regulated dams, of which 678 are classified as high-hazard potential dams. The high-hazard potential classification indicates that a dam may cause loss of life if it were to fail. Of the state’s 678 state-regulated, high-hazard potential dams, 386 have emergency action plans in place. State dam safety programs throughout the country, including California, have made a concerted effort in recent years to increase the number of emergency action plans for high-hazard potential dams, with the national percentage increasing from just over 30% in 2000 to 71% in 2014.

ASDSO encourages members of the public to educate themselves on both the benefits of dams and the risks of dam incidents and failures. Residents can determine if they live in a dam failure flood inundation zone by contacting their local emergency management agency or the state dam safety program. ASDSO recommends that people who live near dams familiarize themselves with evacuation routes, make sure all family members know what to do in the event of an emergency and prepare an emergency kit.

More information on staying safe near dams can be found in ASDSO’s informational guide, Living with Dams: Know Your Risks, which the organization developed in conjunction with the Federal Emergency Management Agency.


The Association of State Dam Safety Officials (ASDSO) is a national, non-profit organization founded in 1984 and dedicated to improving dam safety through research, education and communication. Web:

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Lori Spragens
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