A stretch of Tanque Verde Creek looks as if it might be changing gradually from a watercourse to a woodland.
Tall cottonwood trees, bushy shrubs and even a palo verde tree are growing in the creek bed northwest of Tanque Verde Road.
Trees and other vegetation often show up in riverbeds and washes in the Tucson area, but most of the growth is soon scoured away by surging flows after monsoon storms or winter rains.
The growth in Tanque Verde Creek appears to be maturing and spreading - at least for now.
Undated aerial images on a Google maps website show that the creek bed had less and smaller vegetation at the time the images were taken.
"The increase in riparian trees and canopies along Tanque Verde Creek could be due to several factors, but my guess is that the reduced frequency and severity of scouring floods could have something to do with it," said Julio Betancourt, a scientist with the U.S. Geological Survey specializing in plant ecology, hydrology and other disciplines. He worked for more than 30 years in Tucson before taking a new post recently in Virginia.
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Chris Cawein, deputy director of the Pima County Regional Flood Control District, said a contributing factor could be that the stream channel in the area is broader than some other watercourses.
"Once the water gets to a larger area where it can spread a little bit, the water tends to slow down a bit and tends to scour less and cause less damage" to vegetation, Cawein said.
COTTONWOODS THRIVE
Cottonwoods growing in the creek bed might fare better than other plants as groundwater is drawn down over the years.
"The root zones (for some plants) are no longer wet with groundwater, but cottonwoods can put down some pretty deep roots," Cawein said.
Bob Webb, a hydrologist with the U.S. Geological Survey in Tucson, said scientists are "seeing increases of riparian vegetation pretty much across Arizona" - including plants such as cottonwood trees growing smack in the middle of watercourses.
"It's amazing how things can get established in the middle of streams like that and can, through some sort of hydraulic quirk, withstand floods," Webb said.
Contact reporter Doug Kreutz at dkreutz@azstarnet.com or at 573-4192. On Twitter: @DouglasKreutz
