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CU Boulder geology grad students show how boulders influence canyon formation

Anyone who enjoys whitewater rafting in places like the Colorado River owes a debt of gratitude to the enormous boulders that create the foaming undulation known as rapids, and new research appears to shed more light on how these big rocks help shape the towering canyons around them.

Charles M. Shobe and Rachel C. Glade, geology graduate students at the University of Colorado Boulder, argue in an article recently published online in the prestigious journal Geology鈥攁nd scheduled for publication in print in July 2019鈥攖hose same rocky monoliths play a major role in the geologic evolution of such canyons across vast spans of time鈥攑erhaps even more than the channels themselves.

鈥淥ur results imply that the existing, channel-driven models for canyon evolution may be overly simplistic, even when canyons evolve under a steady external forcing,鈥� they write.

鈥淧eople have been interested in river canyons for a long time, how they control the landscape and erosion,鈥� says Glade, whose research focuses on the geomorphology of canyon walls and who completed her PhD last month. 鈥淏ut there is not a lot of understanding about how they work physically.鈥�

The paper is co-authored by the pair鈥檚 faculty advisors, Distinguished Professor of Geology Robert Anderson and Professor of Geology Greg Tucker.

Shobe and Glade created a computer model to understand the complex, two-way interplay between river-bottom boulders and hillsides to determine the course of canyon evolution.

Rivers that flow through 鈥渟oft鈥� geological formations tend to be wide and flat, like the Mississippi or South Platte. But rivers that flow across 鈥渞esistant rock formations鈥濃�攖hose with hard 鈥渃aprock鈥� in the upper layers鈥攖end to form narrow canyons with steep upper hillsides. From a bird's eye view, canyon rims鈥攖he cliffs marking the canyon edge鈥攆orm a bell shape as the canyon widens downstream.

Initially, erosion will carry sediment downstream, eventually loosening large blocks that tumble into the river below. At first, the presence of such blocks tends to slow the process of erosion, which in turn makes hillsides less steep.

鈥淵ou can see that the bigger the blocks are, the more pronounced the bell shape of the canyon,鈥� Glade says. 鈥淭he big blocks slow down a canyon鈥檚 ability to erode over time and play a major role in changing canyon shape.鈥�

However, Shobe and Glade鈥檚 model showed that rather than simply slowing down the process, the presence of large boulders in the channel created a feedback loop with steep hillsides, resulting in an oscillating rate of erosion and canyon evolution.

鈥淭his interplay between channel and hillslope dynamics results in highly variable long-term erosion rates,鈥� they write.

鈥淭he prediction is, if intrinsic features of rock layers govern eventual shape, then the larger pieces the rock fractures into鈥濃�攇enerally speaking, the harder the rock, the larger the block鈥斺�渢he more bell-shaped the canyon is going to end up and the more unpredictable the erosion dynamics will be,鈥� Shobe says.

This spring, the pair were given an opportunity to field-test the model鈥檚 predictions, courtesy of a grant from the Geological Society of America. They traveled to northern New Mexico where they used a drone to photograph canyon walls and boulders in Rio Grande, and are now in the process of creating a 3D map of the area surveyed.

They are testing one of their main model predictions: 鈥淭he size of the boulders corresponds to the steepness of the canyon walls,鈥� Glade says. 鈥淚f there鈥檚 a pile of big boulders, the steeper the walls should be.鈥�

The model allows for the movement of blocks downstream at times of higher flow, but the researchers found erosion marks in the field indicating that boulders have been wedged in place for a long time.

鈥淲hile they can definitely move in large floods, when they are big enough, they can sit there for hundreds to thousands of years,鈥� Shobe says. 鈥淭hat鈥檚 why the size of boulders is so important in shaping rivers.鈥�

Shobe and Glade write that these 鈥渃hannel-hillslope dynamics鈥� are significant enough to outweigh other factors, such as the rate of geologic uplift, 鈥渃alling into question the ability of landscapes to record tectonic and climatic signals or to reach a steady state during this time.

Better understanding how canyons with resistant rock formations are formed has implications beyond geology, Shobe says.

鈥淭he erosion and breakdown of rock is intimately connected with the climate cycle and the balance of CO2 in the earth鈥檚 atmosphere. The rates at which rock is eroded, and sediment is transported, is tied to the climate cycle as well as the long-term evolution of biodiversity,鈥� he says.

It is uncommon for two PhD candidates to publish ground-breaking new research in a prestigious journal.

鈥淲e鈥檙e really pleased,鈥� Shobe says. 鈥淭his collaboration shows that two grad students can get together and come up with something new and unique while learning to collaborate as early-career scientists.鈥�