The Hetch Hetchy Reservoir in Yosemite National Park supplies water to San Francisco and other Bay Area cities. (Lauren Sommer/KQED)
If you live in San Francisco -- or even certain parts of Alameda, Santa Clara and San Mateo counties -- a portion of your drinking water travels over 150 miles to get to your tap.
It's a journey that begins at the Hetch Hetchy Reservoir in Yosemite National Park, a source of drinking water that has a well-known and crystal-clear reputation: It's so clear that it isn't filtered - only treated.
Bay Curious listeners Alex Kornblum, 8, and his dad, Heath Kornblum, were talking about their drinking water when they landed on this question:
How long does it take for water to get from Hetch Hetchy to San Francisco? And how far does it travel?
The Very Beginning
It all starts high in the Sierra. So high that the water isn't water. It's snow.
"The snow that we're talking about is the snow that falls on the Tuolumne River watershed, which is 492 square miles," says Suzanne Gautier, coordinator for citizen involvement for the San Francisco Public Utilities Commission.
That snow melts into the Tuolumne River, and three smaller creeks that empty into the Hetch Hetchy Reservoir.
"On average per year, San Franciscans consume what would be equal to 1 foot of snow covering that Tuolumne River watershed," says Gautier.
To put this into perspective, it takes about 5 feet of snow to fill the whole reservoir. But if we just need 1 foot -- it seems like there’s plenty of backup supply, right? Not always. During the recent six-year drought, there wasn’t enough snow.
"It must have been about 2015 or so," says Gautier. "They were measuring the snow and it was very, very shallow when it should have been very much higher."
At that point, the Public Utilities Commission started asking people to use less water.
The Journey Continues
Bay Curious listeners Alex and Heath are standing in front of a large map of the Hetch Hetchy water system at the Harry Tracy Water Treatment Plant in San Bruno.
Annie Li, a senior engineer at the San Francisco Public Utilities Commission, points to the yellow and brown squiggly lines on the map, revealing our water's path from Hetch Hetchy to the Bay Area. She says the water first leaves Hetch Hetchy through the O'Shaughnessy Dam. Then it travels through a series of mountain tunnels.
That includes keeping the lights on at San Francisco schools and powering Muni light-rail vehicles, streetcars and trolley coaches.
Li says the water travels downhill the whole way (the system is entirely gravity-fed), whooshing through tunnels drilled through solid granite, and pipelines lined with concrete. Think of it as a giant underground water slide -- twisting around mountains and under rivers -- until it arrives at your tap.
"It takes about three days for the water to get from over here," Li says, pointing to Hetch Hetchy on the map, "all the way into San Francisco."
"Only three days?" remarks Alex. "I thought it would take longer than that. Like four or five days, maybe a week."
His dad, Heath, then asks how they figured out that number. "Did you send like some kind of a probe in the water to time it?"
Li says they use flow meters throughout the system to calculate the answer. These meters will tell you how much water is moving through what pipeline.
"So we do a little bit of math and you say 167 miles, moving at 3 feet per second, equals about 83 hours," says Gautier.
'An Average Answer'
"So, 83 hours, that’s the final answer," says Heath, processing the calculations. "It's sort of an average answer."
The travel time fluctuates because water operators are always releasing different amounts of water, depending on how much people use every day -- and every season.
"During summertime people use more water. During wintertime, when it rains a lot, we don’t need to drink as much, or water our lawns, so we use a lot less," says Li.
They also regulate the water due to diurnal shifts in demand since water use changes throughout the day -- like when most of us are taking showers or washing dishes.
All in, water from the Hetch Hetchy Reservoir serves about 2.7 million residents and businesses.
But to Walk?
As Alex and Heath were getting ready to leave the treatment plant, there was still one more question they wanted answered: How long does it take to walk the same distance?
"I walk 3 miles an hour," says Gautier. "So that's, what, 167 miles?"
Heath chimes in to help with the calculations. "If we say 180 [miles] that's divisible by 3. Right? So about 60 hours."
They were actually spot on, according to Google maps. But Suzanne had some qualms.
"You might get there faster walking than the water would get here, but you wouldn't be stopping for sleep," says Gautier. "And if we were walking to Hetch Hetchy, we would be walking uphill. So that 3 miles an hour is going to be more like a mile and a half."