Cogswell

About Cogswell

The primary purpose of Cogswell is flood risk reduction. The dam is designed to control downstream flooding by temporarily storing excess water during storms and releasing it at a controlled rate. The dam is owned by Los Angeles County Department Of Public Works (local government). It impounds the Wfk San Gabriel R near Azusa.

The dam stands 266 feet tall and stretches 585 feet across, creates a reservoir covering 146 acres, has a maximum storage capacity of 8,969 acre-feet, and collects water from a drainage area of 38 square miles. Completed in 1935, the structure is well-established, with more than 75 years of service.

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 satisfactory, indicating no existing or potential dam safety deficiencies are recognized. An emergency action plan is in place for this dam. The most recent inspection on record was 10/15/2025.

At 266 feet, this dam is taller than 99% of all dams in the United States.

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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