Researchers link wildfires in California’s coastal watersheds to the water quality at the beach

As the summer heat bakes the West Coast, the thoughts of many who have lived here for any amount of time will turn naturally to the idea of wildfire. As of writing, California has already experienced almost 5,000 wildfires in 2026 alone, many of them near the coast, or affecting watersheds that lead to the sea. With all the ash, charred material and other debris making it into the waterways and out to the ocean after storms, what are the effects on water quality at the beaches and in the nearshore?

That question has been on the minds of UC Santa Barbara physics professor Jean Carlson and UCLA and Caltech postdoctoral researcher Carl Swindle, authors of a paper that appears in the Public Library of Science (PLOS) Water . While we can intuit that there must be some adverse effect of large amounts of nutrients and debris from burned houses, buildings, infrastructure and landscape being pumped into the ocean, they said, for all the decades of wildfires and subsequent rainy seasons, the systematic impacts on California's coastal watersheds have not been thoroughly studied.

"How do you decide that water quality and water pollution can be attributed to a wildfire when there are so many other potential causes?" said Carlson, who specializes in complex and highly connected systems. Real-world watershed systems involve many, often interrelated variables, and each wildfire comes with its own set of conditions and responses. This makes generalizing across all of California's coastal watersheds a significant challenge.

However, armed with 20 years of data and a handful of case studies, Carlson and Swindle have teased out relationships between California coastal wildfires and their settings, and the potentially harmful bacteria that make their way to coastal waters at the base of the watersheds in which they burn.

"Overall, we see that watersheds dominated by urban areas are associated with more fecal coliforms in the coastal waters following fires," Swindle said, referring to harmful bacteria that come from the gut and feces of warm-blooded animals, humans included. In contrast, less urban areas with more vegetation exhibited more diverse, and typically less harmful coliform bacteria."

The ability to generalize across California's diverse watersheds can give managers in the state's diverse coastal municipalities better tools to plan for potential beach closures and other measures, such as freshwater supply purification and what to do with debris in the wake of wildfires, according to the researchers.

"If you're at a beach following a major storm in a recently burned area, and you know certain things about the watershed, you might realize that you may be higher at risk due to fecal contamination than if it was a different watershed or had different watershed attributes," said Swindle, a UCSB earth science alum who is conducting his postdoctoral research at the B. John Garrick Institute for the Risk Sciences at UCLA and the Division of Geological and Planetary Sciences at Caltech. This information could also play into longer-term planning that would dictate, for instance, where infrastructure in the watershed could most safely be built.

According to Swindle, "A next step that builds on this work will be to test a wider variety of pollutants and water quality conditions associated with wildfires, such as harmful algal blooms." Ultimately the researchers plan to construct an integrated platform that leverages all this information across even wider coastal spans.

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