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Gully washers and boulder rollers: How monsoons shape the desert

Arizona's desert is sculpted by summer storms — and the small ones add up.
July 7, 2017

ASU geomorphologist talks about the types of floods that do the most to sculpt the land — and it's not the ones you'd expect

Editor's note: This is the first in our weeklong monsoon series. To read the second story, about social media's influence on storm coverage, click hereFor the final story, looking at monsoons from a Native American cultural viewpoint, click here.

Fly above the desert and the paradox is obvious: It’s a place with almost no water, but it’s entirely shaped by water. Washes that flow maybe every 20 or 30 years are etched into hardpan far from mountains.

Monsoons define the desert by driving physical change. Summer storms sculpt the land. Sediment is deposited. An old channel becomes silted up. A new channel is carved in another place. Like the creative process, some of it is a mystery.

Arizona State University geomorphologist Kelin Whipple studies how the Earth is shaped by climate, tectonics and surface processes. One time he was examining an enormous boulder at a ranch near Bagdad, Arizona. The rancher called the storm that moved it a “boulder roller.”

A storm that tosses around boulders the size of minivans doesn’t come around often. People who live close to the land sit up and pay attention to events like that. “They know they’re quite rare,” said Whipple, a professor in the School of Earth and Space Exploration.

Those rare floods chisel the land more dramatically, but it’s the gully washers, not the boulder rollers, that do more work year in and year out.

“A day of high flow is still just one day out of 365,” Whipple said. “A flood that occurs on an annual basis is just not a big flood. Floods that really do the work of deepening canyons and transporting sediments, those are floods that occur at a minimum once every two years. ... It’s the smaller ones that add up. We don’t know really in a quantitative way where is the range of floods that do the most work, either in a mountain environment or a desert environment — we don’t know that very well.”

In the 1950s geologists began to learn that the floods that do the most work over time are the smaller biannual floods.

“They don’t do all that much, but it happens pretty frequently,” Whipple said. “A hundred-year flood does an awful lot, but it happens so infrequently that two-year events add up to more.”

Annual monsoon flooding doesn’t do much at all. It takes critical velocity plus depth to move large material, like gravel. (In geology, gravel is anything larger than sand: granules, pebbles, cobbles and boulders.) Certain thresholds of shearing stress, force that causes layers or parts to slide upon each other in opposite directions, need to be topped before anything happens.

“You need a certain amount of shear stress before you’re going to get gravels actually moving,” Whipple said. “Usually a two-year (flood) will get gravels moving so you get some change. If you’ve got a system that’s armored with a bunch of big boulders, or if you’re asking what floods can actually rip in and deepen the canyon, erode deeper over time, it’s got to work its way into rock and might have higher shear stresses and pull out big blocks, then you’re talking about a five- or 10-year flood to even exceed the shear stress and get anything to happen.”

There are two big rain events in Southwestern deserts: the monsoons and occasional hurricanes wafting up from the Gulf of California. Geologists have a limited amount of data to work with. It only goes back 20 or 30 years, or at most 100. Existing data hints at a tapering-off of flood size. Scientists know how big floods get, but not how often they happen. Floods that would wipe Phoenix or Tucson off the map don’t happen at all.

“They just don’t get much bigger than X,” Whipple said. “It never happens. There’s no such thing as a 5,000-year flood.”

Top photo by Nicholas Hartmann (Own work) [CC BY-SA 4.0 (], via Wikimedia Commons

Scott Seckel

Reporter , ASU Now


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ASU faculty find kindred community of educators in a far-off land

ASU faculty find community on other side of the globe.
July 9, 2017

Lindy Elkins-Tanton, Bert Jacobs lead 2-day teachers' workshop in Tanzania to help create students passionate about learning

Sometimes Arizona State University’s mission is carried out far beyond the boundaries of campus.

This summer two ASU faculty members — Lindy Elkins-Tanton, director of the School of Earth and Space Exploration, and Bert Jacobs, director of the School of Life Sciences — went to Tanzania, where they led a team conducting a two-day teaching workshop for 102 secondary school teachers.

What they found was, despite being on the other side of the globe in sub-Saharan Africa, they are part of a global community.

“What warmed my heart and made my hair stand up is they all want the same thing we want for our students,” Elkins-Tanton said. “We want our students to become passionate, connected, energetic, fair people who solve problems and have a sense of how to do it. ... We really are a global community with a purpose.”

Jacobs, co-founder of a nonprofit called HEAL International that has been teaching HIV/AIDS prevention in rural Tanzania for 10 years, arranged the workshop.

“Most of our work revolves around service learning,” Jacobs said. “We take ASU students to Tanzania, and they work with African students and teach basic public health and AIDS / HIV prevention.”

This is the first year HEAL International has done a teachers’ workshop.

“Since we had Lindy; myself; her husband, James; her son, Turner — all established people — we felt like we had a better chance of having an impact on teachers,” Jacobs said.

The June workshop was co-sponsored by Beagle Learning, a learning platform for managing online discussion- and question-based classes founded by Elkins-Tanton; her husband, James, a mathematician and teacher; and her son, Turner Bohlen, a technologist.

“We have this vision that I hold in common with my position at ASU and Beagle Learning: We want to help create a next generation of students who are passionate about learning,” Elkins-Tanton said.

She worked up a curriculum and a math lesson with her husband. HEAL International advertised for teachers to attend and found a workshop space. They warned her that people might not show up or that only a few might show up.

The teachers walked down dusty roads to a village and a market to buy paper and pens and found a little copy shop. The tables were set with supplies.

“To our great thrill, everyone showed up,” Elkins-Tanton said. “At first they were very shy.”

The team gave two one-day workshops, teaching math learning, HIV prevention and leadership.

“It ended up really positively,” she said. “I think it was a big success.”

“Most of the stuff that came up was the same stuff we hear in the U.S. what we want for our students,” Jacobs said. “It was quite remarkable.”

After the workshop, the team traveled to Dodoma, the capital, for a meeting with the minister of education.

“It was a short meeting, and it was very productive,” Jacobs said. “The ministry is interested in us expanding what we’ve been doing for 10 years and expanding it to other regions of Tanzania.”

Government officials want to ramp up STEM education in the country.

“They really think it’s their future,” Elkins-Tanton said.

The meeting had the potential to spark a new international partnership, Jacobs said.

“I think the potential is much larger to include all of ASU and what we do to help work with Tanzania to build their education system,” he said. “In my mind it’s critical.”

Elkins-Tanton has some education funding from NASA as part of her Psyche mission that could bring ASU online learning into Tanzania.

“I’m feeling grateful for being part of ASU,” she said. “The whole university really cares about this kind of thing and having this kind of connection.”

Scott Seckel

Reporter , ASU Now