On July 15, 2020, a quite ordinary channel was engulfed in nauseating dust with an ominous black sludge. Cañada del Oro in Arizona became choked by this monster after a light rain while the Bighorn Fire raged nearby. A video posted on Twitter by Pima County officials captured the fast-moving fallout, dark with ash and soot.
The Bighorn fire was caused by lightning. Since June 5, the fire has engulfed 48,377 hectares of national park in a region dominated by diverse ecosystems from saguaro cacti to pine and spruce forests. Firefighters have brought it under control, but the fire is still burning in the foothills of the Catalina Mountains.
Earlier we wrote that forest fires can be useful, but not in this case. Fires not only burn the landscape, but also pose a danger during future rains. Forest fires leave the land charred, barren and unable to absorb water. But even light rain can lead to devastating flash floods and mudslides. Fire alters soil structure by mineralizing organic matter and releasing metals and toxins that normally cannot be washed away by water. The new soil structure repels water.
«Mudflows in burned basins require much less rainfall than unburned areas,» explains California’s California Center for Water Science. In Southern California, as little as 7 millimeters of rain in 30 minutes caused a flood.
With the loss of soil-holding vegetation, ash and loose earth choke waterways, lowering dissolved oxygen levels, while increased nutrient levels stimulate cyanobacterial growth and algae blooms, which take up even more oxygen.
A possible lack of oxygen is suffocating fish, crabs and other aquatic animals, leading to massive deaths similar to those that occurred after Australia’s unprecedented 2019-2020 summer bushfire. If fish and other wildlife somehow survive all of this, they could starve to death, unable to see their food, as muddy water reduces visibility and deprives aquatic plants of the light they need for photosynthesis.
Sediments can also seep into dams and threaten our drinking water, making mud too thick for filtration systems. When large solid debris also joins the fast moving sludge, it contributes to erosion along the entire flow path, posing a risk to water treatment infrastructure.
Fast moving destructive mudflows caused by intense rainfall are one of the most serious post-fire hazards. Mudflows like these are especially dangerous because they tend to happen without much warning.