Why parts of San Francisco Bay turn pink every year

If you’re on a flight to San Francisco International Airport in the early fall, you may spot pink patches of water while landing that weren’t there before. The patches are salt ponds, and the colors are caused by the organisms that live in the salty waters.

“At different times of the year, different microbes grow in different salinities. There’s different kinds of algae and other things like that, and the different ones grow in different salinities,” Dave Halsing, executive project manager at the South Bay Salt Pond Restoration Project, told SFGATE. “So sometimes they’re pink, sometimes they’re orange, sometimes they’re sort of blue.” 

While the ponds might seem natural, the various hues in the water are caused by salt farming. Cargill, an American salt supplier, farms on the bay using solar production, according to the Cargill website. The seawater captured in the pond is harvested once the water reaches 25% salinity and the salt crystallizes. 

“Throughout the year, [the salt] concentrates. The water gets more concentrated, more salty. The water evaporates away, and so it gets saltier and saltier, and then at the end, they harvest the salt,” Halsing said.

While it might not be as aesthetically pleasing, the environmental impacts are incredibly beneficial, Halsing said. Returning the land to marshes helps support several species of fish, birds and rodents, some of which are endangered, including the salt marsh harvest mouse and the southern steelhead trout. The marshes also absorb wave energy, storm surges and high tides, which protects from floods as sea levels rise. 

“Tidal marsh wetlands in San Francisco Bay were the dominant habitat in the bay before development and, depending on exactly the time frame you look at or what boundary you want to put on it something like 80 to 95% of those have been lost … so that’s quite a lot of the natural habitat that’s gone,” Halsing said.

The restoration process doesn’t happen overnight. Halsing said each pond’s reversion process can take from five to more than 20 years. 

“It depends a lot on how deeply subsided the bottom of the ponds are. So some of them, the marsh will only grow when it’s at the right elevation relative to the tides,” Halsing said. “… And then, of course, it depends on how fast sea level rise happens too, which is a new thing that, when we started this project, we knew it was a thing. We knew it was going to happen. We didn’t know it was going to be this bad this soon, so we kind of have to get ahead of it a little bit.”