Rain triggers a flood of steaming muck near Arizona wildfire. Officials warn it can happen again

(CNN)Emergency officials are warning people to watch out for flash floods in areas scorched by Arizona's Bighorn Fire.

Officials in Pima County released video last week of a debris flow in Tucson's Cañada del Oro Wash after a light rain. They said wildfires increase flood risk because they leave the ground charred, barren and unable to absorb water.
At first, the advancing floodwaters look like a slow-moving, black blob moving toward the camera like a creature in an old-fashioned monster movie. The water rushes past in seconds, carrying ash and steaming debris.
Debris flows like this are one of the most dangerous hazards after a wildfire because they can occur with little or no warning, according to the US Geological Survey's California Water Science Center.
At last report, the Bighorn Fire had burned 119,741 acres since it was started by a lightning strike on June 5. Authorities said it was 90% contained. No homes or other structures were burned in the fire, CNN affiliate KOLD reported.
Pima County officials said the fire increased the risk of flash floods and mudflows in the foothills of the Catalina Mountains and urged residents living downstream of the burn area to take steps to reduce that risk.
"Loss of vegetation and physical changes to the soil leave burned areas prone to increased runoff and erosion," the county warned on its website. "This combination can cause flash floods and mudflows to flow faster and with more volume than normal flows during a typical monsoon storm."