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Pine Mountain Tanks project underway on Mt. Tam Watershed

Pine Mountain tank project

Aug. 24, 2023

Watershed visitors, Ross community advised to plan for construction-related road, trail impacts

Marin Water has begun construction activity to replace use of the Pine Mountain Tunnel water storage facility with two new 2-million-gallon water storage tanks. The project will increase water storage reliability in Ross Valley, boost firefighting capabilities, reduce water loss and improve water quality.

The project involves the installation of a pair of modern, seismically sound 2-million-gallon concrete water tanks along the west side of Concrete Pipe Road near Bolinas and Sky Oaks roads, as well as the decommissioning of Pine Mountain Tunnel – a 100-year-old former water conveyance apparatus that has been used since 1971 to store water in a remote portion of the Mount Tamalpais Watershed. This project is a two-phase effort, which will span multiple years.

The first phase of construction involves rough grading work and the installation of a soil nail retaining wall in preparation for the construction of the storage tanks. This work will continue through the end of January 2024, and then pause for the winter and spring of 2024 before beginning again in the summer of 2024. The first phase is expected to be complete by the end of January 2025. Phase 2 work will then follow and includes the installation of the pre-stressed concrete storage tanks.

To help facilitate construction activity and ensure the safety of watershed visitors and construction workers, a number of Watershed road and trail traffic controls and/or closures are planned or are already in place. Additionally, beginning in September, truck-hauling activity related to the project may generate traffic impacts for some Ross community members for several weeks.

Long-term Watershed road/trail access interruptions in place through January 2024 and August 2024 – January 2025

  • Traffic controls are in place along Sky Oaks/Bon Tempe/Bullfrog roads.
  • Concrete Pipe Road closed Monday – Friday, 8 a.m. to 5 p.m. at Bolinas-Fairfax Road
  • Motorist parking prohibited along Bolinas-Fairfax Road at Concrete Pipe Road.

Additional short-term Watershed road/trail access interruptions in place through October 2023

  • Construction truck traffic traveling through the watershed has closed many additional Watershed fire roads and trails that intersect with the truck route Monday – Friday, from 8 a.m. - 5 p.m. through the end of October 2023.
  • A full map of the current watershed road and trail closures is available on Marin Water’s project website to help watershed goers plan their visits.  

Temporary traffic impacts in town of Ross from Sept. 5 – Oct. 15, 2023

Truck-hauling activity related to the construction may generate traffic impacts for community members in the Town of Ross beginning Sept. 5 and lasting thru Oct. 15, 2023.

  • The truck route will travel through the Mt. Tam Watershed, past Natalie Coffin Greene Park and through Ross via Dibblee/Lagunitas Road to Sir Francis Drake Boulevard on weekdays from 9 a.m.- 5 p.m. with daily accommodations planned to allow for school drop-off and pickup times.
  • Traffic control flaggers will be stationed at Glenwood Avenue and at the Natalie Coffin Greene Park parking lot to guide vehicular and bicycle traffic during hauling activity.
  • The construction traffic through Ross will travel in only one direction. The hauling trucks will return to the construction site via Bolinas-Fairfax Road. 
  • A map of the hauling route through Ross is available on Marin Water’s project website.

Anyone traveling near areas of the construction should allow for extra time, use caution around heavy equipment and adhere to all road and/or trail closure signs. 

Built just seven years after Marin Water was established, the 1.5-mile Pine Mountain Tunnel was constructed in 1919 to convey water from Alpine Reservoir to Fairfax and San Rafael. The tunnel has been used as a water storage facility for more than half of its life, as advancements in water pumping technology negated its original purpose: to move water to surrounding communities using gravity.  Replacing the tunnel with modern storage will enhance the District’s water system resiliency by improving seismic safety, reducing water loss and increasing water quality. The project also provides for an increase in emergency water storage on the Mount Tamalpais Watershed to bolster firefighting capabilities, and increases water storage capacity in Ross Valley by 30%, ensuring greater storage reliability for Marin Water customers.

More information about the project is available online at