The sound of rock shattering echoed across the dry channel in the Tonto National Forest as Jorge Santiago, a 23-year-old geology student, drilled a 10-inch hole in the limestone rock that once cradled a stream.
Mr. Santiago was among the researchers from the University of Arizona who earlier this year inserted pressure transducers, bullet-shaped objects that can be used to measure water levels, into the walls of the dry streambed.
They were not expecting the water to return to the stream. Instead, they were using the transducers to measure something else entirely: the possibility that water-laden rocks, soil and debris could barrel downhill from hundreds of miles away after rain falls on an area burned by wildfire.
It’s a scenario that emergency managers across the world are increasingly preparing for. As nations continue to burn fossil fuels and generate the greenhouse gases that are heating the planet, the risks of both extreme wildfire and major rain events are rising.
In Southern California this week heavy rains raised concerns of landslides sweeping over fire-scarred terrain. On Tuesday officials issued evacuation orders for more than 100 homes near the 2024 Airport Fire burn scar and the Palisades Fire that struck in January.
The orders were lifted by Tuesday evening, but the threat shows the long-lasting dangers of wildfires.
Hundreds of miles away from Los Angeles in a pine forest in Arizona, Luke McGuire and a team of researchers spent the summer gathering data to better understand the threats posed by post-fire disasters.
Their work comes as research shows that post-fire debris flows could become larger, more destructive, and more common on a hotter Earth, according to a study published last year.
Ann Youberg, the environmental geology chief at the Arizona Geological Survey, first installed a pressure transducer after the 2017 Pinal Fire, a fire caused by lightning in a strand of forest in southern Arizona. Over the last seven years they’ve installed more than 40 pressure transducers, which each cost about $500. They lose about 20 percent, Dr. McGuire said.
Data from the pressure transducers is recovered after a debris flow, which can travel at speeds of up to 100 miles an hour, annihilating neighborhoods. In the Southwest, more than 90 percent of debris flows happen during the first two years after a wildfire.
Researchers return to the site, and by chain saw, shovel and sheer force, they try to retrieve the rammed-in transducers and upload their data. That information could lead to better modeling of how and where a deadly landslide could strike.
“As we see fires expanding into new areas across the Western U.S., they’re happening in areas where we’ve never collected any data,” Dr. McGuire said, “so all of our models are extrapolating.”
Researchers from the University of Arizona and the geological survey are trying to add a couple of monitoring sites every year to their portfolio.
Their research is becoming more important as fires get bigger and hotter, leaving severely burned soils in their wake. At the same time, a warmer atmosphere holds more moisture, dropping heavier rain over the singed soil.
That combination of intense burn and intense rain, plus steep slopes, means post-fire disasters could become more dangerous in parts of the Pacific Northwest and the Southwest that have few defenses, like concrete basins carved into the foothills surrounding Los Angeles that can catch debris before it reaches homes.
Post-fire hazards like flash floods and debris flows are most likely to occur in areas that have seen moderate to high levels of burning, where the soil is so charred it can’t absorb more water.
While debris flows are increasing across the globe, they’re more common in places with more fires, more rain and more mountainous terrain.
Brendan Murphy, a geomorphologist at Simon Fraser University, who once worked with Dr. Youberg, has documented an increase in debris flows in British Columbia, where extreme wildfires have been spiking since 2015.
The scars from the 2021 Lytton Fire, which burned more than 206,000 acres and a tiny mountain town reliant on tourism, produced roughly 300 debris flows, destroying portions of the Trans-Canada Highway and producing more post-fire hazards than any one wildfire in California, according to Dr. Murphy.
With grants from Canada’s Forest Service, he’s started to install monitoring equipment after major fires. With his students, he raked through satellite images of old fires and scoured unpublished government reports looking for historic evidence of debris flows.
While an official report in 2020 noted 36 debris flows, by August of 2024, Dr. Murphy had found more than 1,400 post-fire debris flows.
“A lot of this increase has to do with the changing fire size and fire severity, but also the convergence of a severe wildfire and then an atmospheric river,” Dr. Murphy said, referring to a patch of very wet air that moves in from the Pacific before hitting the mountains and dumping heavy rain.
In addition to the pressure transducers, researchers install rain gauges that measure the intensity of rainfall.
Dr. Youberg has been working on ways to help local governments identify strategies to mitigate the threat from a debris flow.
“Let’s figure out where the problems are before the fires start,” Dr. Youberg said as she hiked through the Tonto.
The growing awareness of debris flows in places like Southern California can also create anxiety among local residents every time it rains.
“Our job has gone from not just telling people where the problems might be after the fire, but just as importantly, where and when we don’t have to worry about it,” said Jason Kean, a landslide hazard researcher for the United States Geological Survey.
After the devastating fire on Maui, officials asked the landslide hazards program to assess the risk of post-fire debris flows in Lahaina, but because there were no steep slopes nearby, Dr. Kean and his colleagues were able to reassure residents.
“I doubt we would’ve been asked to do that 10 years ago because nobody was thinking about it,” Dr. Kean said. “But now they do.”
Austyn Gaffney is a reporter covering climate and a member of the 2024-25 Times Fellowship class, a program for journalists early in their careers.
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