A camera points to an apparently benign trail in Pima County, Arizona. But only a few moments in the video, something dark appears in the distance, before covering the dry land. Eventually, it resolves into a black river of stick and mud coal, flowing almost as fast as clear water.
The video, posted on Twitter by Pima county officials, shows a debris flow on July 15 after a forest fire in Cañada del Oro Wash, a drainage canal, and a mountain bike trail in the county. According to the United States Geological Survey (USGS), fires can change the way rainwater flows over land. Under normal circumstances, most dirt is able to absorb a lot of water, preventing flash floods from occurring each time it rains. But after a forest fire, the land can no longer absorb as much water. And even minor rains can lead to flash floods full of debris.
Who had this on their 2020 hellscape bingo card? pic.twitter.com/fUNvIVS7aw July 16, 2020
“Rapidly moving, highly destructive debris flows from heavy rain are one of the most dangerous post-fire hazards,” according to the USGS California Water Science Center. “Much less rain is needed to trigger debris flows from burned basins than from unburned areas. In Southern California, just 7 millimeters (0.3 inches) of rain in 30 minutes has caused debris flows.”
In 2018, landslides and debris flow after the Thomas fire killed 13 people in two California cities after a winter storm, according to the Santa Barbara Family Life.
I just returned from this exact location to check and see what the wash looks like today … Authorities say this year, the risk of flooding is higher due to # BighornFire. Thanks to Billy, a nearby resident, who gave me permission to enter the area! @KVOA https://t.co/sJ77BIW7uz pic.twitter.com/LHJjdo9JV317 July 2020
According to KGUN 9, a local news station, this recent debris flow appears to have originated from the Bighorn Fire, which has burned approximately 120,000 acres (485 square kilometers) in the area.
No casualties or major damage have been reported from this event, despite the dramatic images. But the risk of large floods filled with debris of this type remains for years after a fire, according to the USGS. The biggest events usually happen in the first storm season after a big fire.
To prevent major debris disasters after the fire, the USGS examines burned regions for potential risk and establishes flood indicators to provide emergency alerts if flood waters pass quickly.
As the climate changes, forest fires have become more common worldwide, bringing with them all kinds of risks, according to NASA. Research released in June by the National Bureau of Economic Research suggests that wildfires burn four times as much land each year in the United States as they did 40 years ago.
Originally published in Live Science.
