Every year, millions in the Bay Area rely on water flowing through the Mountain Tunnel from Hetch Hetchy Reservoir in the Sierras. Designed in the early 20th century, this tunnel is vital for both drinking water and hydroelectric power. To ensure its continued function, the San Francisco Public Utilities Commission's (SFPUC) Water Enterprise conducts annual maintenance and upgrades.

The critical work happens during the winter when the tunnel is drained, typically from December to February. During this period, water is supplied from reservoirs in the East Bay and Peninsula. The winter of 2024-2025 marks the fourth consecutive year of repair and enhancement efforts.

SFPUC crews access the tunnel through “adits,” entry points distributed along its length. These adits are being expanded and improved for better access. This year, a new 1,000-foot adit near the Priest Reservoir was constructed, providing essential access for repairs. Other adits support ventilation and supply lines for workers.

One major improvement was the installation of two large double disc knife gate valves at the bottom of the tunnel’s flow control facility. Once fully installed, these valves and the accompanying new building will help regulate water flow, reducing wear on the tunnel’s concrete lining.

Another key achievement this year was completing repairs to the tunnel’s deteriorated concrete lining, caused by decades of water flow. SFPUC crews worked around the clock, bringing machinery and supplies deep into the 20-mile-long tunnel. The repairs included grouting the gaps between the concrete lining and the original rock surface, which required setting up a grout plant high in the Sierras and pumping grout deep into the tunnel.

This work, critical for the tunnel’s long-term stability, will continue in the 2025-2026 winter outage when further grouting will take place. All of these efforts were completed with meticulous planning and oversight, ensuring the tunnel remains operational for future generations.