Pia Mill Reservoir

About Pia Mill Reservoir

The primary purpose of Pia Mill Reservoir is irrigation. The dam stores and diverts water for agricultural irrigation, supporting farming operations in the surrounding area. The dam is owned by Eric A. Knudsen Trust (private). It impounds the Waihohonu Stream Offstream near Koloa.

The dam stands 17 feet tall and stretches 450 feet across, creates a reservoir covering 6 acres, has a maximum storage capacity of 38 acre-feet, and collects water from a drainage area of 0 square miles. Completed in 1910, 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 poor, meaning the dam has recognized safety deficiencies that require remediation to address risks. An emergency action plan is in place for this dam. The most recent inspection on record was 01/29/2026.

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 Hawaii dam safety program can provide more specific information about emergency planning for this structure.

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