Anderson Dam Tunnel Project on track for completion in 2026

A view of the tunnel entrance on the right, and the diversion outlet structure. The staged, 13-foot diameter pipe will be placed inside the tunnel and will connect to the diversion outlet structure. April 2026.

The Anderson Dam Tunnel Project is entering its final stages, with Valley Water contractors completing key milestones such as the final concrete work inside the tunnel and installation of critical components. The project remains on track for completion in 2026.

This project plays a vital role in Valley Water’s broader effort to rebuild Anderson Dam and improve safety for downstream communities. In 2020, the Federal Energy Regulatory Commission directed Valley Water to reduce Anderson Reservoir to “dead pool,” or as low as possible, until the dam’s seismic upgrades are complete. In response, Valley Water launched a series of six projects to support the draining of Anderson Reservoir. These projects are all on track to be finished before work on the dam begins.

At the center of this effort lies the Anderson Dam Tunnel. The new tunnel will greatly improve Valley Water’s ability to release water from the reservoir when needed, including during dam construction. The existing outlet pipe is just four feet in diameter, limiting how quickly water can be moved. In contrast, the new system features a 13-foot-diameter pipe, significantly increasing capacity to keep the reservoir empty during construction. Once operational, this new tunnel will enable Valley Water to release water more quickly during storms or emergencies.

Construction on the tunnel began in 2021, and crews completed excavation of the more than 1,700-foot-long tunnel in 2024. Since then, they have focused on building out the tunnel’s interior. Most recently, crews completed the final concrete pour inside the tunnel, marking an important structural milestone. In April, workers began installing a large steel pipe within a section of the tunnel. Following installation, the annular space between the pipe and the tunnel will be backfilled with concrete to create a strong, durable system designed to withstand seismic activity.

Progress is also visible outside the tunnel. Crews have completed armoring the Northern Channel and construction of the North Channel weir, which features a labyrinth (zigzag) design. This innovative structure helps water flow more safely and efficiently from the reservoir into the North Channel of Coyote Creek. By folding the weir into a zigzag pattern, engineers increase its length without widening the creek, helping reduce erosion, improve habitat and enhance flood protection.

Construction of the South Channel weir, a smaller-capacity structure, is also complete. During higher flows, water will rise and be directed to the North Channel weir, where the design helps slow and safely manage the flow. Together, these features improve water conveyance through Coyote Creek during reservoir releases.

The Anderson Dam Tunnel Project is one of six coordinated efforts that support safe reservoir operations while the dam is being rebuilt. The other related projects include:

Coyote Creek Flood Management Measures: Completed in 2025, this project is the first phase of Valley Water’s effort to improve flood protection along Coyote Creek. Crews built more than 8,500 feet of floodwalls along a four-mile stretch between Old Oakland Road and Interstate 280. This work increases the creek’s ability to handle more water when it flows from the nearly finished tunnel at Anderson Dam during storms or emergencies. Learn more here.

Cross Valley Pipeline Extension: Completed in 2024, this project ensures that Coyote Creek and the Coyote Percolation Pond in South San José have enough water. This helps recharge groundwater and supports the surrounding habitat and wildlife during dam construction. The pipeline extension allows us to send additional imported water into Coyote Creek and the percolation ponds while we rebuild Anderson Dam.

Coyote Percolation Dam Replacement: This project replaced an outdated steel flashboard dam with a modern inflatable dam, providing greater flexibility to manage creek flows near Metcalf Ponds. The new system allows operators to maintain groundwater recharge during low flows and safely pass higher volumes of water during storms. It also supports increased releases from Anderson Reservoir—up to five times greater during major storm events—ensuring Coyote Creek can safely handle higher flows while protecting groundwater basins, water supply, and local wildlife. The dam is anticipated to be operational by the fall of 2026.

Coyote Creek Stream Augmentation Fish Protection Measure: Also called the Chillers Project, this effort aims to protect fish in Coyote Creek. It includes a water-cooling system installed at the Coyote Pumping Plant in Morgan Hill. The system cools water taken from San Luis Reservoir before is released into Coyote Creek. Keeping the water cool helps protect native fish species, including federally threatened steelhead, which rely on cold-water habitat to survive and thrive during the construction of Anderson Dam. The project is scheduled to be completed by the fall of 2026.

Live Oak Restoration Reach Project: This project will install fish habitat improvements in a 2,600-foot-long reach of Coyote Creek at Live Oak Park Area. Improvements will include installing 36 large wood structures and placing more than 10 tons of gravel and rocks for spawning and rearing. The project is scheduled to start on June 1, 2026, and will be completed on Dec. 31, 2026.

These projects serve as a critical precursor to the Anderson Dam Seismic Retrofit Project, which will rebuild the dam to meet modern safety standards. Construction on the retrofit project is expected to begin in January 2027 and will include major improvements to the dam embankment, spillway, and outlet works.

As the tunnel project moves closer to completion, it represents a major step forward in ensuring the long-term safety and resilience of Anderson Dam.

Side-by-side images at the end of the tunnel. On the left, the yellow waterproofing and steel reinforcement in progress in September 2025. On the right, a 2-foot-thick section of concrete encasing the reinforcement was completed in February 2026.

Side-by-side images of heavily reinforced articulated joints within the tunnel, which will function as a seismic protection system during an earthquake. On the left, concrete was placed for the tunnel floor invert. On the right, concrete is placed for walls, arches, and colored concrete in the articulated joint gaps.

A view of the 13-foot diameter pipe, waiting to be installed. Workers began installing the pipe in April 2026.

A view of one of two fixed cone valves, measuring 11 feet in diameter, located on each side of the diversion outlet structure. December 2025.

An aerial view of the North Channel weir (left), the South Channel weir (bottom), the diversion outlet structure, and the concrete retaining wall securing the hillside around the tunnel entrance.

Valley Water manages an integrated water resources system that includes the supply of clean, safe water, flood protection and stewardship of streams on behalf of Santa Clara County's 2 million residents. Valley Water effectively manages 10 dams and surface water reservoirs, three water treatment plants, an advanced recycled water purification center, a state-of-the-art water quality laboratory, nearly 285 acres of groundwater recharge ponds and 333 miles of waterways. We provide wholesale water and groundwater management services to local municipalities and private water retailers who deliver drinking water directly to homes and businesses in Santa Clara County.