Mountain Tunnel, key piece of Bay Area water infrastructure, getting major overhaul

GROVELAND, Tuolumne County -- Just beyond the Don Pedro Reservoir, nestled in the foothills of the Sierra Nevada, lies key infrastructure in supplying much of the Bay Area with water.

However, some of the infrastructure is old, like the 100-year-old, 19-mile-long Mountain Tunnel, which is a key conduit for the Hetch Hetchy water flow on its route to the Bay Area. But a project is underway to restore and repair the Mountain Tunnel.

"It's a large project. But, we're able to rehabilitate it, instead of having to build a brand new tunnel," said Randy Anderson, the Project Manager for the Mountain Tunnel Improvements Project.

Construction started in 2021, and the project is about 40% complete, says Anderson.

Katie Miller, the Director of Water Capital Programs for the San Francisco Public Utilities Commission, says neglecting these improvements wouldn't bode well.

"85% of the water supply for 2.8 million people in the Bay Area comes from the Hetch Hetchy Reservoir, and all of that water passes through the Mountain Tunnel," she said. "If this tunnel failed or collapsed, it would take us probably more than a year to repair it, which would mean we wouldn't be able to provide water to the Bay Area. That would be a catastrophic situation."

The construction includes providing a new tunnel access location, repairing tunnel lining, restoring tunnel flow capacity, and creating a new flow control facility that can help improve the tunnel's longevity.

Work inside the actual Mountain Tunnel can only occur during a short period of time during the winter when the tunnel is out of service and empty of water. This was the second of what will be five anticipated shutdowns of the tunnel, as a result of the project.

"When we're doing work on the Mountain Tunnel, we have to drain the entire Hetch Hetchy water system," Miller said. "We need to use local water supplies during that time. We have local reservoirs in the East Bay and on the Peninsula - the Crystal Springs Reservoir and the Calaveras Reservoir."

Anderson says they've made good progress so far and hopes the project will give this component of the water infrastructure that the Bay Area relies on a century of stability.

"Our goal is to get another 100 years out of this rehabilitation project," Anderson said.

The project is expected to be finished in the winter of 2026.