World in a Grain of Sand: USU Geologist Explores Utah's Arroyo Cycles

In July 1883, Utah’s Deseret News newspaper recounted chilling details of “The Great Washout,” a catastrophic flood on Kanab Creek that took out water supply dams and farmland and carved a gaping, muddy gash through town:

Masses of earth as large as common houses floated down the stream [Kanab Creek] with willows still standing. Extensive damage to crops, and all farming land in the canyon was destroyed. Some cattle killed. Canyon near old city dam was cut 50 feet down and 16 rods [88 yards] wide.

“This event, and others like it that occurred around the same time in this region, were very dramatic and disruptive,” says Tammy Rittenour, assistant professor in Utah State University’s Department of Geology. “In one area, the creek incised 100 feet, making it difficult for ranchers to get their cattle to water or to irrigate their fields.”

As in Kanab, large floods caused by spring snowmelt and thunderstorms with heavy rains sliced through mid-sized streams throughout southern Utah, leaving deep, flat-bottomed channels with steep walls of sediment known as arroyos.

“Inhabitants of those communities, mostly Mormon settlers struggling to establish farms and ranches, must have thought Doomsday was coming,” Rittenour says.

Kanab and neighboring communities were in the midst of the cutting phase of an arroyo cycle. It’s these cycles Rittenour is investigating as a 2011 grant recipient of the National Science Foundation’s prestigious Faculty Early Career Development (CAREER) program.

“An arroyo is an entrenched river, like a canyon cut into sediment,” she says. “It’s similar to a gully or a gulch, but much larger. You can’t jump across an arroyo.”

Though once-in-a-lifetime events, the arroyo cutting episodes that took place in the American Southwest from about 1865 to 1915 aren’t unique on a geologic scale. Scientists surmise that similar events have occurred for millennia, the most recent taking place between 600 to 800 years prior to the 19th and 20th century accounts.

Between periods of rapid incision, arroyos appear to follow prolonged aggradation or “filling” periods. The question is: What’s driving these cycles of rapid entrenchment followed by slower rates of sedimentation and infilling?

Three main hypotheses have emerged to explain this phenomenon, Rittenour says. Some scientists point to land use changes, such as sudden population settlement with attendant grazing and farming that led to removal of native vegetation and other human-driven impacts. Others contend that climate change is the driving mechanism behind the cycles. Still others propose that arroyo cycles are simply a natural, geological occurrence.

She plans to test these hypotheses with a multi-faceted approach.

“We’re developing a detailed and well-dated stratigraphic record of past cut-fill cycles at six adjoining semi-arid drainages in southern Utah using optically stimulated luminescence (OSL) and radiocarbon dating,” says Rittenour, who serves as director of the USU Luminescence Lab. “We’ll also complete geomorphic mapping and GIS and hydrological analysis of these watersheds.”

Secondly, she and her student team will investigate processes that drive aggradation by characterizing sediment production and erosion rates. Finally, the team will use hydrological tools and tree-ring chronologies to try to decipher the driving forces behind rapid arroyo entrenchment.

“As we examine the different arroyo sections — located near the upper Paria River, the Virgin River, Kanab Creek and the Escalante River — we’ll hold as many factors the same as possible,” she says. “We want to know whether or not these incision episodes have happened synchronously. That will yield important clues about what’s causing these cycles.”

Rittenour says she’s excited about using single-grain OSL dating, a specific technique that allows researchers to determine the ages of individual grains of sand. The USU Luminescence Lab, established in 2007, is one of just a few labs in the country with this dating capability.

“Single-grain OSL dating enables more accurate age estimates and allows for more systematic sample collection than radiocarbon dating,” she says. “OSL samples can be collected from nearly any sediment containing sand, while material for radiocarbon dating can be sparse in these systems.”

However, Rittenour says she and her team will employ a combination of OSL, radiocarbon and dendrochronology – tree ring counting – to obtain the most accurate ages dates possible and thus determine the timing of past arroyo cycles.

“Our study will shed a whole new light on arroyo questions by taking us thousands of years into the past,” says Rittenour, who credits her success in securing funding for her project, in part, to her participation in the USU Office of the Vice President for Research’s NSF CAREER grant writing workshop in 2010. “It’s important for us to discover what’s causing these cycles, so we can predict what the future holds.”

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Contact: Tammy Rittenour, 435-213-5756, tammy.rittenour@usu.edu

Writer: Mary-Ann Muffoletto, 435-797-3517, maryann.muffoletto@usu.edu