Anderson Dam Seismic Retrofit Project

Anderson Dam holds back Santa Clara County’s largest reservoir, and since 2020 it has sat nearly empty. Federal regulators ordered Valley Water to draw the reservoir down over fears the 240-foot earthen dam could fail in a major earthquake, cutting off a water source for much of Silicon Valley. The fix is one of the biggest dam safety projects in the country: tear the existing embankment down to bedrock and rebuild it to modern seismic standards.

Project Scope
The roughly $1.9 billion program has two parts. The first, already built, is a 1,700-foot tunnel driven through the hillside to serve as a permanent low-level outlet, letting operators release water and manage the reservoir during and after construction. FlatironDragados bored that tunnel, with AECOM handling design and GEI Consultants supporting construction management. The second and far larger phase rebuilds the dam itself, excavating the old 1950s fill and replacing it with an engineered embankment sized to survive a magnitude-7 event on the nearby Calaveras Fault. PMA Consultants supports program scheduling.

Why It Matters
Anderson normally stores about ten times more water than any other reservoir in Valley Water’s system, so keeping it offline strains supplies for nearly two million people. Every year the dam stays drained, the county leans harder on imported water and groundwater. The retrofit also carries flood-risk weight: a failure would send a wall of water toward Morgan Hill and San Jose neighborhoods downstream. Getting the dam back to full capacity restores a big chunk of local storage right as California swings between deeper droughts and heavier atmospheric-river storms. Construction is expected to run well into the next decade before the reservoir can refill.

Project Team & Details