The recent monsoon storms have caused burn-scar debris and ash from last summer’s massive Bighorn Fire to run off in various Tucson washes, darkening the water.
Water flowing this week in the Pima and Cañada del Oro washes contained some ash from the 2020 fire on Mount Lemmon and in the Catalina Mountains, said Brian Jones, division manager at the Pima County Flood District.
“There hasn’t been a rain event, yet, that’s really caused any severe issues on the washes that are coming out of the mountain, but they have been enough to get things wet,” Jones said Thursday.
“The last couple of days, the rain has been more focused over the low end and not so much on the mountain. If we get one big storm or a number of ... storms over the mountain, then everything is going to be fairly wet, so the likelihood of significant runoff coming off of the Bighorn burn scar is higher at this point.”
Last summer, videos popped up on social media of of black sludge with bunches of tree limbs moving down the Cañada del Oro wash as a result of the fire damage. While we haven’t seen that this year, Jones said the rains have similar intensity as last year’s and there is still more debris in the mountains.
“Assuming that we get more intense rainfalls and larger rainfall up on the mountain,” flooding might have the capability of “moving some of the larger dead burned trees and bushes,” Jones said.
Usually, the soil and vegetation can soak up the water and slow the water down. However, the severe fire wiped out vegetation, which increases flow velocities, erosion and volume of water coming down from the mountains, causing heavier flooding.
Jones says drivers should turn around or wait out floods when there’s water across roads. The post-fire flows can be even more hazardous because of the debris, including sticks and logs.