By any measure, the fires that tore through Northern California were a major disaster. Forty-two people are dead, and 100,000 are displaced. More than 8,400 homes and other buildings were destroyed, more than 160,000 acres burned—and the fires aren’t all out yet.

That devastation leaves behind another potential disaster: ash. No one knows how much. It’ll be full of heavy metals and toxins—no one knows exactly how much, and it depends on what burned and at what temperature. The ash will infiltrate soils, but no one’s really sure how or whether that’ll be a problem. And eventually some of it—maybe a lot—will flow into the regional aquatic ecosystem and ultimately the San Francisco Bay.

That’s the bomb. Here’s the timer: An old, grim joke about California says that the state has only three seasons: summer, fire, and mudslides. Those mudslides happen because of rain; the Santa Ana (or Diablo, if you’d prefer) wind-driven wildfires of autumn give way to a monsoon season that lasts through winter and into spring. The rains of 2016–2017 ended a longstanding drought and broke all kinds of records.

Scientists and environmental health agencies know, mostly, what to expect from ash that comes from burned vegetation. But these fires included something a little new. They burned through the wildland-urban interface and into cities. “For how many structures that were burned in fairly small areas in these fires, I think that's a first-of-its-kind event,” says Geoffrey Plumlee, associate director of environmental health for the US Geological Survey. “The concern is, can they get it cleaned up before the heavy rains come?”

Chemically, ash is fascinating. If the fire that made it burned at lower than about 840 degrees Fahrenheit, it’ll be darker-colored, maybe even black, and mostly organic carbon. At higher temperatures the carbon burns away, leaving inorganic compounds of stuff like calcium, magnesium, and sodium, and it’ll be whiter and fluffier. Even hotter fires, like above 1,100 degrees, leave nothing but oxides. Inside a single fire, combustion happens at different temperatures in different places, and because ash is so light, wind can transport it, so the composition of ash from the same fire can vary.

So depending on how combusted the ash is, it’ll have different chemical compositions. And that’ll mean the ash will mix either better or worse with underlying soil. Water won’t stick to more hydrophobic ash, so rainfall might run off faster, carrying away the surrounding soil as sediment. More hydrophilic ash might mix into the water and wash into nearby streams.

Now, carbon is the backbone element of organic systems. Having some flow off of burned hillsides and into aquatic ecosystems isn’t necessarily bad. It’ll increase what’s called “primary productivity,” allowing algae to reproduce, which means more food for fish. “Naturally occurring, lower-severity fires can have positive impacts,” says Kevin Bladon, a forest ecohydrologist at Oregon State University. The fires free up organic carbon and put nutrients like nitrogen and phosphorous into play. “But the really large, high-severity megafires that we’ve started to observe push the systems in a lot of cases too far.”

That means dangerously large algal blooms, so-called eutrophication that can eat all the dissolved oxygen out of a waterway, making it unlivable for everything else. Sediments freed up by the hydrophobic ash end up making water more turbid—bad for people if that water’s supposed to end up coming out of someone’s tap, and bad for fish because sediment can gum up feeding and breeding grounds.

The problem’s getting more familiar as an after-effect of fires ranging from Australia to Canada to the US. Climate change makes fires and storms more severe. As one of Bladon’s papers notes, the 2002 Hayman Fire in the Colorado Rockies sent 765,000 cubic meters of sediment, ash, and other stuff into Denver’s drinking water reservoirs, and the problems were still there four years later. “We’re anywhere from five years to 100 years in terms of the longevity of effects,” Bladon says. “That really depends on the severity of the fire and our ability to get some vegetation re-established on site.”