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Beginning January 8th through March 11th the District will have a water source (supply) interruption, of our main water supply from Hetch Hetchy Reservoir through the Mountain Tunnel, due to routine scheduled repairs that will be performed by the San Francisco Public Utilities Commission (SFPUC). During this time, we will be using a water treatment system that produces excellent quality water, but has less capacity than our normal system so we ask our customers to conserve water to the maximum extent possible to preserve water supply for drinking, sanitation and fire protection.  The District will be posting roadside water conservation signs around the community with tunnel interruption dates as reminders.  Much more information on the GCSD water sources, reason for the source water interruption and the alternate water source can be found below or by calling (209) 962-7161.

During the tunnel outage, the District uses its Microfiltration Trailer while pumping water from the District’s secondary water source, which is Pine Mountain Lake. The Filter Trailer runs at one-quarter capacity of the District’s two Water Treatment Plants, therefore the need for water conservation by our customers.

 A fantastic video from inside the tunnel was recently published by SFPUC.  

The Mountain Tunnel Improvements Project was approved by the SFPUC to improve the condition of the existing Mountain Tunnel (tunnel), and to ensure the tunnel’s continued ability to provide quality drinking water reliably to its 2.7 million customers in the Sierra Foothills and San Francisco Bay Area.


The tunnel was constructed between 1917 and 1925 and has been in service for more than 90 years. Inspections in 2017 showed deterioration of the tunnel lining and other deficiencies. These deficiencies result in reduced flow the, increased groundwater intrusion, and increased turbidity. The deficiencies also diminish the ability to provide drinking water reliably to customers, and increase the difficulty of performing maintenance in the tunnel during normal operation.


The tunnel, approximately 19-miles-long, has been providing the most pristine water in the state of California since 1925 as part of the Hetch Hetchy Regional Water System. It transmits drinking water originating from Hetch Hetchy Reservoir through Kirkwood Powerhouse, where it generates hydropower, to Priest Reservoir downstream. Water flows entirely by gravity though this tunnel, which is unlined upstream for over 7miles and has an unreinforced concrete lining for 11 miles in its downstream section.


San Francisco Public Utilities Commission has been making interim, urgent repairs to Mountain Tunnel until this larger proposed project can be designed and constructed. Crews thoroughly inspected Mountain Tunnel in January 2017. During that winter crews performed initial repairs of the tunnel linking as well as improvements to above ground facilities to widen access routes for current and future repair crews. This winter, the urgent repairs identified in 2017 will be completed.