Heavy rains fell in New Mexico over the weekend after two weeks of wildfires, causing severe flash flooding and mudslides near Ruidoso.
Floodwaters blackened with soot and ash from the South Fork and Salt fires sloshed down mountain canyons and into town, turning Highway 70 into a river and overturning fuel tankers, according to videos posted to social media. Homes and businesses were damaged, and emergency responders reported 77 water rescues.
“It's going to be a long road to recovery,” said Kelly Gladden, spokeswoman for the village of Ruidoso. Monsoon season typically starts around the Fourth of July, but this year it coincided with two weeks of wildfires, greatly increasing the risk of flooding. “We're going to continue to see these conditions with every significant rainfall,” Gladden said.
The South Fork and Salt Fires killed two people and burned more than 25,000 acres last month, but the burn scars they leave behind could pose an even greater danger to residents than the fires themselves.
Climate change, driven primarily by the burning of fossil fuels, continues to cause an increase in both more intense fires, which kill vegetation and dry out soils, and more extreme precipitation, which brings more rain in a shorter period of time. The combination of dry soils and heavy rain increases the likelihood of hazards such as dangerous flash floods and mud flows, which mix water, mud, rocks and trees in the aftermath of a fire.
“It's a mistake to think of the floods and debris flows after the fire as a footnote, rather than a big part of the fire itself,” said Don Falk, a professor of natural resources and fire ecology at the University of Arizona. “They can be more destructive than the fire and cause more loss of life.”
On Saturday afternoon, after authorities said the wildfire was under control, Brittany Smith, 34, was helping her parents return to their cabin when suddenly, a new emergency alert rang on her parents' phone: a flood warning and an emergency evacuation order.
That afternoon, a 6-and-a-half-foot-high wall of black water slammed into their neighborhood in the steep-sided Upper Canyon. On Sunday, as families tried to return, the village of Ruidoso issued a third evacuation warning: “Evacuate now!” the order read.
Three factors increase the likelihood and risk of post-fire floods and debris flows: how badly the soil is burned, the amount of rainfall, and the steepness of the terrain.
Plants in the canopy and forest floor normally act like sponges to absorb rainwater, which is especially important during the intense monsoons that occur in the Southwest summer.
But this sponge effect is destroyed by super-hot fires, and when rains come, the dead soil shifts rapidly, making steep slopes unstable.
The effects could last for years. “The fact that fire intensity has increased over the past few decades has led to increased post-fire hazards,” said Luke McGuire, an associate professor of geomorphology at the University of Arizona.
Karen Miranda Gleason, spokeswoman for the Burn Area Emergency Response Team, said that while official burn severity maps have not yet been released, areas of the South Fork Fire will likely see high soil burn severity.
Over the past 150 years, land management practices have generally minimized natural burning and planned burning (the practice of intentionally starting small, controlled fires as a preventative measure).
TJ Clifford, a BAER team leader at the Department of the Interior, said New Mexico's fires would not have burned as badly if the region had been maintained using land-management techniques such as thinning forests and setting deliberate fires. But that may be unpopular.
“Planned fires put smoke in the air, and people don't like smoke in the air,” he said. “It's very hard to get support.”
Floods have already battered the region, but debris flows remain a threat. Clifford said a flood is like dragging a silk gown down a waterway, while a debris flow is a type of landslide that's like rubbing sandpaper down a waterway, washing away anything it hits.
“Post-fire debris flows are different to floods,” Dr Maguire says: they cause different problems, have more severe impacts on people and infrastructure, and often affect areas outside typical floodplains.
Dr Maguire and his colleagues published research in Nature Reviews Earth and Environment in May showing that post-debris flow disasters are becoming more frequent: 68 percent of the places in the world that have already seen debris flows are likely to experience new ones in the future.
While Smith and her parents' homes have so far escaped damage, their neighbors have not been so lucky. Burnt trees line the edges of their flooded driveways, while across the street, a chimney made of river rock towers over a house destroyed by fire. “We're just overwhelmed with emotions,” Smith said Sunday. “The Upper Canyon is devastated.”
The official cause of the fire is still under investigation. The FBI is offering a $10,000 reward for information leading to the arrest of those responsible for starting the fire.