A look inside 1,750-foot tunnel under construction beneath Anderson Reservoir

Mark Sayre
·2 min read

MORGAN HILL, Calif. - The scenes from 2017 are unforgettable, with firefighters rescuing residents along Coyote Creek in San Jose from their inundated homes.

Heavy rains that year overwhelmed the Anderson Reservoir, sending water over the dam's spillway into Coyote Creek. In 2020, the federal government ordered the entire reservoir to be drained due to seismic safety concerns.

Now, a massive tunnel is under construction beneath the Anderson Reservoir.

"So this new tunnel will help us keep the reservoir empty as we approach the larger Anderson Dam seismic retrofit project which will include a rebuild of the dam embankment itself," said Ryan McCarter, the project manager of the Anderson Dam Seismic Retrofit Project.

When complete, the tunnel will be more than 1,750 feet long and will have the capacity to drain Anderson Lake reservoir about 15 times faster than what is possible with the current dam.

"In the end, when we are completed with the project, it will be used as an emergency outlet only. So it is a much larger amount of water that we would be able to release in an emergency such as an earthquake or a flood event – a large storm," McCarter said.

The new tunnel, capable of accommodating two lanes of vehicle traffic, will serve two purposes. First, it will enable the complete drainage of Anderson Reservoir, which currently holds about 3 percent of its total capacity, allowing for the removal of the current dam.

Then, once the new dam is built, the tunnel will provide far more capacity to quickly remove water from the reservoir.

"If this project had been in place in 2017 we would not have seen the flooding we did there," said Supervisor Cindy Chavez.

The cost of the overall project – including the new dam – is about $2.3 billion.

"Anderson dam will create thousands and thousands of jobs. These jobs are also pathways for apprenticeship programs, for women, for veterans and for youth," said Jean Cohen, executive officer of the South Bay Labor Council.

Anderson reservoir is by far the largest in Santa Clara County and is a key link for water storage and also a long-term reliable water supply.

The entire project is scheduled for completion in 2032.