People often wrap themselves in a hug and shiver when I tell them I swim in the San Francisco Bay without a wetsuit in winter. Yes, it’s cold, but that’s the pleasure of it—my mood rises as the temperature drops.
Next they shudder about the water quality. “Isn’t the Bay dirty?” they ask.
That’s a good question, especially in the rainy season. Along most of the Bay’s 400 miles of shoreline, downpours wash trash, oil, dog poop, and anything else in the gutter down storm drains and creeks that flow straight into the Bay. When I take a walk in the rain, I cast a swimmer’s critical eye on the greasy runoff pouring down the street, assessing how much it will get diluted once it flows into the Bay.
The good news is that the dilution is massive, with tides pouring in from the Pacific twice a day and, from the other direction, huge quantities of water from the Sierra Nevada sluicing in through the California Delta.
Swimming in the Bay
A series by Susan Kuramoto Moffat on what you can learn about our local waters from diving in.
See her Summer 2025 Issue column for resources on how to get started swimming outside.
The result is that the water of the Bay is on the whole shockingly clean. The reality year-round is much better than the popular perception. Even in winter, Bay water is mostly safe to swim in. It smells good. It tastes fine. Many Bay beaches get A and A+ scores for water quality from the environmental nonprofit Heal the Bay—some even in wet weather.
This is something of a miracle, considering that the Bay is one of the most urbanized estuaries in the United States, surrounded by more than 7 million people flushing their toilets every day, not to mention factories, ports, refineries, and miles of dirty asphalt.
Even in winter, Bay water is mostly safe to swim in. It smells good. It tastes fine.
As I immerse my head in the water that runs from cities into the Bay, I am grateful to nature for the Bay’s enormous daily tidal flux. I also appreciate the efforts of humans.
I give thanks to the public servants who run the East Bay Municipal Utility District wastewater treatment plant four miles south of my home beach. I thank the scientists tracking pathogens and even the bureaucrats who are requiring me to spend an eye-watering sum to replace my home’s apparently leaking sewer lateral.
I thank the Save the Bay movement that helped people see the Bay as more than a dump, and California’s 1969 Porter-Cologne Act, which helped lead to the federal Clean Water Act. San Francisco Bay didn’t go from a stinking, hazardous mess in the 1960s to the swimmable beauty it is today by accident. Before the Clean Water Act passed in 1972, raw sewage went straight into the Bay. Fifteen years later, most water around Bay beaches was deemed swimmable in summer and today many are safe in winter, too.
Admittedly, a few Bay beaches are persistently filthy in wet weather. San Francisco, alone among Bay Area cities, does the massive work of treating most of the runoff from its streets. But in heavy downpours its combined wastewater-stormwater system gets overwhelmed and millions of gallons of the combined flux spills into the Bay.
And the Bay still has serious problems with toxic contaminants. As a swimmer, the infinitesimal amounts of PCBs, heavy metals, and “forever chemicals” that I could potentially absorb through my skin are likely inconsequential. But these toxins accumulate in sediment and in animal flesh up the food chain. I swim in the water but don’t eat fish from it regularly.
Nutrients including nitrogen from sewage, even after it’s treated, are also an issue. Although harmful algal blooms have not generally been a huge problem in San Francisco Bay, the reddish Heterosigma akashiwo bloom that killed thousands of fish in 2022 raised concerns.
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Scientists are studying whether the Bay might be growing more algae-prone, in part because humans are making it clearer—that is, less muddy. Urban development and dams on tributaries of the San Joaquin and Sacramento rivers steal essential sediment from the creek and river water that feeds the Bay, so more light is available to the photosynthetic algae, fueling blooms.
A turbid Bay is a healthy Bay. It’s supposed to be brownish—suspended sediment is essential to limiting excessive algae growth, just as it is to forming and maintaining wetlands. And the animal plankton that also adds to the Bay’s opaqueness isn’t dirty, it’s life. When I can’t see my feet as I wade into the water, I’m not thinking ick, I’m wondering, what microscopic barnacle and crab babies are in the soup around my ankles?
Maybe we need to adjust our image of a clean, healthy Bay. It’s not meant to look like the Caribbean or Hawaii. It should be the color of green tea mixed with chai, with nicely muddy, marshy edges—and lots of people swimming happily in the clean, biologically rich murk.
It’s recommended to stay out of the water for 72 hours after a significant rain.
Water quality reports on select beaches are available here:
