What The Hell Is This River Of Black Sludge Oozing Through Arizona?

Post-fire debris flow at Cañada del Oro Wash, Pima County, Arizona. Credit: Official Pima County

If you’ve spent lockdown binging on '90s disaster movies, cackling as the lava pours improbably down the street, or fissures open in the ground, you’re going to love this. Officials in Arizona have shared a video of a dark, steaming river of what looks like sludge oozing down a trail in Pima County, with a caption many of us have asked over the last few months: “Who had this on their 2020 hellscape bingo card?”

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The video, posted by Pima County officials to its social media channels, was taken on July 15 at the Cañada del Oro Wash, a drainage channel at the northern county line following a "minor storm". That ominous-looking, fast-moving dark mass is a flash flood of mud and debris following wildfires in the region. It may look cool in an apocalyptic kind of way, but the video was shared with the hashtag #FloodsFollowFires as a stark warning to show how quickly flash floods can appear and move. 

"This video was taken Wednesday, July 15, 2020 in the Cañada del Oro Wash on the northern County line following a minor storm," officials wrote on Facebook. "Even a light rain can produce devastating flash floods and mudflows, often with little warning."

Wildfires actually raise the risk of flash floods, as the ground becomes charred, dry, and unable to absorb water. This means that even light rain can trigger devastating floods and mudflows, which are not only fast-moving but pick up debris – silt, rocks, even trees – along the way, causing potential damage and destruction wherever the flow takes it. Until the ground recovers and vegetation grows back, these runoffs can occur for years aferwards.

It's likely this flash flood is runoff from the Bighorn Fire that has been burning at the west end of the Santa Catalina Mountains north of Tucson, Arizona, since June 5. Named after the local Bighorn sheep, the blaze has now moved south-east, towards the Catalina foothills. Thought to be caused by a lightning strike, it's estimated to have burned 48,377 hectares (119,541 acres) so far but is mainly contained now.

Post-fire debris flows can also destabilize and erode land, paths, roads, and anything in its way as it flows downhill, so don’t go looking to take your own shaky-cam film footage in the hopes of starring in your own disaster movie, it might get a little too real and ‘90s Bruce Willis isn’t around to rescue you.

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