May 15, 2025

Corte Madera – Construction is set to begin in June on Phase 2 of a multi-year project that will install a pair of 2-million-gallon storage tanks on the Mount Tamalpais Watershed. 

Located near Sky Oaks and Bolinas-Fairfax roads, the Pine Mountain Tanks project will eventually replace the Pine Mountain Tunnel for water storage, bringing about a host of benefits in the process. 

“The launch of Phase 2 marks a major milestone for this generational infrastructure project,” said Marin Water Board President Matt Samson. “This capital investment not only strengthens Marin Water’s overall system reliability, it also expands system capacity for our Ross Valley customers and provides additional water storage for firefighting — both now and for generations to come.” 

Currently, water for Ross Valley is stored in the Pine Mountain Tunnel, which was constructed in 1919 to convey water from Alpine Reservoir to Fairfax and San Rafael. The tunnel was converted to a water storage facility in 1971 and remains in that use today. 

In its place, Marin Water is installing a pair of 2-million-gallon, pre-stressed concrete tanks – each 45 feet in height with a diameter of 90 feet. The cost of the project is approximately $23.2 million and is funded through Marin Water’s Capital Improvement Program. 

Once installed and integrated into the District’s water supply infrastructure, the Pine Mountain Tanks will bolster the District’s water quality, boost firefighting capabilities on the watershed, increase water system capacity for Ross Valley and improve water supply resiliency due to improved seismic stability compared to the existing Pine Mountain Tunnel. 

Phase 2 of the Pine Mountain Tanks Project comes after two years of work to clear space for the tanks, including the construction of a 560-foot concrete retaining wall that reaches 75 feet at its highest point. Like the wall, the concrete storage tanks will be stained a natural brown to blend in with the rugged hillsides of the surrounding Mt. Tam Watershed environment. 

Construction will continue through January 2029, with seasonal pauses for weather and environmental factors. During construction, road and trail access near the construction site will be limited. Those traveling to the Watershed are encouraged to review Watershed visitor notices, and to heed all construction-related signage as work commences. 

For more information about this project, visit marinwater.org/PMTTanks.