This Bizarre River of Black Sludge in Arizona Is Totally Real

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. 

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 afterward.

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.

Source - https://www.iflscience.com/environment/what-the-hell-is-this-river-of-black-sludge-oozing-through-arizona/