Mackay

About Mackay

Mackay is a dam in Custer, Idaho. The dam is private-owned. It impounds the Big Lost River Sinks near Mackay.

The dam stands 75 feet tall and stretches 1,430 feet across, creates a reservoir covering 1,392 acres, has a storage capacity of 45,000 acre-feet, and collects water from a drainage area of 788 square miles. Completed in 1918, 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 10/04/2019.

At 75 feet, this dam is taller than 97% 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 Idaho dam safety program can provide more specific information about emergency planning for this structure.

Search for dams near your address to see all dams in your area with hazard classifications and safety data.