Lake O'neill Dam

About Lake O'neill Dam

The primary purpose of Lake O'neill Dam is fire protection, stock, or small fish pond. The dam maintains a small impoundment used for fire suppression, livestock watering, or as a fish pond. The dam is owned by Dod Usmc (federal). It impounds the Santa Margarita Riv Offstream near 22 Area -chappo.

The dam stands 17 feet tall and stretches 1,500 feet across, creates a reservoir covering 115 acres, has a maximum storage capacity of 1,395 acre-feet, and collects water from a drainage area of 9 square miles. Completed in 1883, the structure is over a century old.

This dam carries a high hazard potential classification, which means that a failure or uncontrolled release of water would likely cause loss of human life. This classification reflects the consequences of failure, not the likelihood — it indicates that people live or work in the area downstream. Its condition is rated fair — the dam has minor deficiencies that are being addressed or do not pose an immediate safety threat. An emergency action plan is in place for this dam. The most recent inspection on record was 06/23/2017. That was 8 years ago, which is on the longer end for a high-hazard structure.

Living Downstream

If you live or work downstream of this dam, consider taking these steps: learn your local evacuation routes, check whether your county emergency management office has a dam failure inundation map, and sign up for local emergency alerts. The California dam safety program can provide more specific information about emergency planning for this structure.

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