Construction was completed in 1925 on the 19-mile-long Mountain Tunnel

San Francisco and its southern neighbors depend on the San Francisco Water Supply – an ancient water system – for its daily water. Known as Hetch Hetchy – it is the name of a valley, a reservoir and a water system. Back in 1923, the O’Shaughnessy Dam was completed on the Tuolumne River, flooding the entire valley under the Hetch Hetchy Reservoir.The dam and reservoir are the centerpiece of the Hetch Hetchy Project, which in 1934 began to deliver water 167 miles (269 km) west to San Francisco and its client municipalities in the greater Bay Area and supplies 2.6 million Bay Area residences and businesses.

The water is delivered via the Mountain Tunnel, a key part of the Hetch Hetchy water system – is at risk of a “catastrophic collapse” and will cost more than $100 million to repair or up to $630 million to replace, according to the San Francisco Public Utilities Commission (PUC).

City officials have known for 25 years that significant work is needed on the 19-mile-long tunnel just outside Yosemite National Park in a steep, hard-to-access wilderness area. They considered making it part of the PUC‘s decade-old, $4.6 billion water system improvement program, which is now more than 80 percent complete. But ultimately, the 89-year-old connector was left out of the rebuild, which focused on upgrading Bay Area water facilities that could fail in an earthquake.

PUC officials say that the program gave priority to infrastructure on the three major Bay Area fault lines whose failure could shut off the water supply, such as the Calaveras Dam near Fremont – and that the tunnel, as well as other “upcountry” projects, didn’t pose enough of a seismic risk to be included.

Now, the PUC is grappling with whether to shore up the Mountain Tunnel, which would require shutting it down for two months at a time for up to 10 years, or go the far more expensive but arguably more reliable route of building a new tunnel. The issue has taken on more urgency since the January release of a report that laid out the options and recommended building a new tunnel.

Will the San Francisco water supply continue unabated? Time will tell.

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