Rancho Seco

About Rancho Seco

The primary purpose of Rancho Seco is tailings. Tailings dams contain waste material from mining operations — typically a slurry of finely ground rock left over after valuable minerals have been extracted. These structures require ongoing monitoring because tailings can contain heavy metals and other contaminants. The dam is owned by Sacramento Municipal Utility District (public utility). It impounds the Tr Hadselville Cr near Clay Twonsite.

The dam stands 58 feet tall and stretches 3,400 feet across, creates a reservoir covering 165 acres, has a maximum storage capacity of 2,950 acre-feet, and collects water from a drainage area of 1 square miles. Completed in 1972, the structure is well into its service life at over 50 years 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 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 09/29/2025.

At 58 feet, this dam is taller than 95% 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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