Sinlahekin Dam No 1

About Sinlahekin Dam No 1

The primary purpose of Sinlahekin Dam No 1 is recreation. The dam creates a reservoir used for recreational activities such as fishing, boating, and swimming. The dam is owned by Wa Dfw (state). It impounds the Sinlahekin Creek near Loomis.

The dam stands 14 feet tall and stretches 180 feet across, creates a reservoir covering 83 acres, has a maximum storage capacity of 333 acre-feet, and collects water from a drainage area of 64 square miles. Completed in 1949, 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 10/19/2016. That was 9 years ago, which is on the longer end for a high-hazard structure.

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

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