Tunnel Vision

The Golden Gate Bridge and San Francisco-Oakland Bay Bridge are Northern California's most famous monuments to America's golden century of engineering (bookended roughly from the Civil War to the Apollo missions).

For my money, however, the most important is the Hetch Hetchy system, which furnishes water to the Bay Area. [bold added]

The regional water system provides water to 2.4 million people in San Francisco, Santa Clara, Alameda and San Mateo counties. Eighty-five percent of the water comes from Sierra Nevada snowmelt stored in the Hetch Hetchy reservoir situated on the Tuolumne River in Yosemite National Park. Hetch Hetchy water travels 160 miles via gravity from Yosemite to the San Francisco Bay Area.

The system uses gravity until the water reaches its destinations, where pumping stations are required. With its three hydroelectric power plants Hetch Hetchy produces electricity sufficient to power 300,000 households per year.

Groveland access point to Mountain Tunnel (Chron)

Essential to the entire system is the 19-mile Mountain Tunnel, which is undergoing repairs:

Today, the 19-mile-long Mountain Tunnel is critical to San Francisco: It is the only pathway for Hetch Hetchy Reservoir water to travel from one of the powerhouses that generate hydroelectricity to another reservoir downstream.

For two consecutive winters, Mountain Tunnel has been under renovation after it was first red-flagged for major repairs 30 years ago. SFPUC has completed about 40% of the total construction and plans to close the tunnel for five more winters, ending in 2027...

Through pipes above and below ground, water from Hetch Hetchy Reservoir flows 150 miles through a series of tunnels, powerhouses and reservoirs before reaching the Bay Area’s local water system 32 hours later. Taking advantage of grade changes from the Sierra Nevada to the Bay Area, the original engineers were able to construct a probable pathway for water to travel entirely via one of the most climate-friendly sources of power available: gravity.

I am glad that we still have enough engineering knowhow to repair the Mountain Tunnel at a cost of $238 million instead of replacing it for $500 million to $1 billion.

Also, the SFPUC is betting that local reservoirs will be sufficient to satisfy the lower seasonal demand when the Tunnel is closed for repairs for the next four winters.

So pray for rain.