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River networks slice through landscapes to find symmetry

by Lindsay Jolivet
Mar 17 / 14

The world’s river basins are more unstable than previously thought, according to a study published March 7 in Science; they shift position over time, jostle with other river networks and gobble up their neighbours in search of equilibrium.

Topography of the Blue Ridge Escarpment, Virginia and West Virginia. Atlantic draining rivers at low elevation (blue colors) are advancing westward, capturing the older west-draining rivers (yellow and reds) of the Appalachian Plateau.
Image courtesy of Scott McCoy

The study detailed a method of studying “water divides” – ridges in the landscape that split up neighbouring river basins – to predict if networks were stable or if they were in the process of shifting. It was led by CIFAR Senior Fellow Sean Willett (ETH Zürich) and co-authored by colleagues including CIFAR Fellow Taylor Perron (Massachusetts Institute of Technology) and postdoctoral researcher Scott McCoy (ETH Zürich, MIT), who is supported by CIFAR.

The researchers discovered the new mapping technique while studying river networks and then applied it to rivers in the Loess Plateau in China, the eastern Central Range of Taiwan and the southeastern United States. Their method incorporates geometrical rules about how rivers move into a model that calculates stability based on measurements from each side of a river basin.

The researchers were surprised to find restless river activity almost everywhere.

“We found one example of a stable landscape but pretty much everything else we looked at was far from equilibrium,” says Willett, a member of the Earth System Evolution program.

“This included landscapes that we thought of as being relatively old and stable.”

Even the southeastern United States, which hasn’t been disturbed by major tectonic activity for almost 200 million years, has moving rivers. By contrast, only China’s Loess Plateau is nearly stationary.

Within the unstable networks, there is a slow but constant struggle for power between rivers.

“Neighbours will push and pull and shove and fight for space,” Willett says.

For example, if one river steals a branch from another nearby, it also starts to capture any rainfall on its new territory, growing even stronger from the extra volume of water.

“An aggressive river that captures area from a victim gains more strength from the victim and therefore may continue to capture more of the victim or another of its neighbours,” he says.

“The poor victim or loser may lose drainage area to the point that it simply disappears.”

These battles carve out landscapes, influencing whether areas will have gentle rolling slopes, ridges or valleys. Rivers also carry nutrients and minerals along their changing trajectories, depositing natural resources and affecting where the soil will grow rich for agriculture.

Willett says that by understanding the geometry of these natural processes, researchers can better understand how various terrains, their economic resources, and their wildlife evolved to where they are today. While unmoving river networks would prevent aquatic species in different rivers from communicating with each other, reorganization leads to greater biodiversity.

“If instead the landscape is dynamic — there are captures of drainage area from one river into another — then this is the way that aquatic species can traverse geographically and thereby mix genetic pools.”

Willett says the development of the new technique began as a shared interest between himself, McCoy and Perron.

“It really emerged rather organically as a discovery,” Willett says.

“I think looking for these serendipitous discoveries is a big part of why CIFAR works, because a lot of it is just about putting people together and letting them talk.”