Three Mile Reservoir (l.&c.)

About Three Mile Reservoir (l.&c.)

The primary purpose of Three Mile Reservoir (l.&c.) is irrigation. The dam stores and diverts water for agricultural irrigation, supporting farming operations in the surrounding area. The dam is owned by Grant Williams (private). It impounds the Three Mile Creek near Helena Valley.

The dam stands 70 feet tall and stretches 470 feet across, creates a reservoir covering 3 acres, has a maximum storage capacity of 300 acre-feet, and collects water from a drainage area of 5 square miles. Completed in 1926, 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 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 10/20/2021.

At 70 feet, this dam is taller than 96% 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 Montana 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.