Monsoon rains transform the desert, nourishing the plants and wildlife with much needed water. But, these rains can also transform the landscape.
Scientists are working to better understand how the monsoon affects our landscapes.
One of the effects comes as debris flows. They occur when water-saturated soil in upper elevations gives way in a landslide. As it proceeds downhill, the soil mixes with boulders, vegetation, silt, water and other debris forming a thick slurry, which threatens roads, homes, utility lines and other infrastructure in its way.
"If they [debris flows] are coming down a canyon where the walls confine them, they do not lose much water," said Phil Pearthree, research geologist at the Arizona Geological Survey. "When they get to an area where they can spread out, they typically drop their boulders first, and the finer grained stuff may go further."
Evidence of past debris flows can be seen dotted around the base of the Santa Catalina Mountains. Some from up to 15 thousand years ago. And, most recently, from 2006 in Sabino Canyon.
Heidi Schewel, spokesperson for the Coronado National Forest, said tons of debris flowed down the slopes at the canyon, taking out bridges, roads and rest areas. In some places, the debris was tall enough to bury a building, she said.
The event at Sabino Canyon was a rare and remarkable occurrence.
"A lot of people saw that the Sabino Canyon they love had changed," Schewel said. "But, Sabino Canyon did what Sabino Canyon should have been doing. It took what came down and moved it through."
Pearthree said that thunderstorms after a fire create enough runoff in the watershed to cause debris flows. So, it is fire, not water, what will trigger more of these events in the future. Wildfires wipe out the vegetation that, normally, holds soils in place.