California's Anderson Dam tunnel project marks milestone as completion nears

(UI) — Valley Water reached a major milestone in its Anderson Dam tunnel project this summer, finishing the final segment of a 1,736-foot tunnel adjacent to the dam. This development brings the agency closer to its goal of enhancing water release capabilities in emergencies.

To complete the tunnel’s last 347 feet, crews used a specialized micro-tunnel boring machine, maneuvering it 30 feet beneath the reservoir’s surface. After the boring was done, divers and construction teams carefully removed the machine from the tunnel’s endpoint using a large crane.

While the tunnel excavation has wrapped up, additional work is underway to prepare for the next phases. Valley Water is focusing on securing the creek channel below the dam, which will handle higher water flows from the new outlet. Crews are also installing a structural lining in the tunnel, reinforcing it for reliable long-term use.

A key component of the project is a large diversion outlet structure between the tunnel entrance and Coyote Creek. With 30-foot-tall walls, this structure is designed to slow water flows as they enter the creek, protecting downstream areas from potential flooding.

Expected to be operational by mid-2025, the completed tunnel will enable greater control over reservoir releases, especially during emergencies. Following the tunnel’s completion, work will shift to a comprehensive seismic retrofit for Anderson Dam, scheduled to begin in 2026. Valley Water is finalizing design, permits, and environmental reviews for this retrofit, which aims to fortify the dam against earthquake threats.

Valley Water emphasizes that the Anderson Dam retrofit remains a top priority. Upon completion, the dam will allow the reservoir to store up to 90,000 acre-feet of water, supporting the needs of nearly a million people annually while enhancing regional water security and environmental resilience.

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