Researchers in Taylor Perron's group at MIT and the Swiss Federal Institute of Technology (ETH Zurich) have developed a mapping technique that measures how much a river network is changing, and in what direction it may be moving. Their results are published in this week’s issue of Science.

Balancing across a divide

Knowing where river systems are shifting may help explain curious disparities found in certain river basins. For instance, Perron points to some river networks where scientists have identified sediment or fish species that better match the geology or genetic makeup of a nearby river basin. The drainage divide separating these basins may have shifted abruptly sometime in the past, transferring one river’s headwaters to the river on the opposite side.

How ridgelines, or drainage divides, migrate is determined by how fast either side of a ridge erodes. If a river on one side cuts into the underlying bedrock faster than the river on the other side of the divide, this imbalance can push a ridge across a landscape over time.

“But that’s pretty hard to gauge,” Perron says. “Measuring these very slow erosion rates, typically tenths of a millimeter per year, is difficult. You can’t go to a river and point your Star Trek tricorder at it and see how fast it’s eroding.”

Instead, the researchers calculated what’s called an “equilibrium elevation” for drainage basins on either side of a divide — that is, how high or low a riverbed would have to be to balance two forces: a river’s erosion into rock, and a region’s tectonic activity, which pushes rock up. A mismatch in equilibrium elevation across a drainage divide means the divide is likely in motion: To reach a stable state, the divide must migrate toward the side with the higher equilibrium elevation.

The researchers found that by mapping the equilibrium elevations of entire river networks and calculating mismatches across drainage divides, they could predict the directions in which divides throughout a landscape are migrating.

Shifting boundaries

The researchers applied their technique to three very different river networks, found in the Loess Plateau in China, the eastern Central Range in Taiwan, and the region between the Appalachian Mountains and the Atlantic Ocean in the southeastern U.S.

As it turns out, the rivers of the Loess Plateau — a tectonically stable geologic region — are essentially stationary, as their equilibrium elevations are closely matched across drainage divides. However, the much younger river networks in Taiwan form a very different pattern, seeming to shift dramatically in response to tectonic activity in the region.

What was most surprising to Perron was what is likely occurring in the southeastern United States. While the landscape, which stretches from northern Florida to Virginia, has not experienced much tectonic activity for hundreds of millions of years, the group’s map suggests that river networks in these areas are on the move. From their results, the researchers find that the Blue Ridge Escarpment is moving inland, and essentially dragging behind it river basins, slowly stretching them across the landscape. Over time, Perron says, larger basins near the coast will take over smaller ones, vastly changing the topography.

“What you’d probably see is some of these smaller coastal drainage basins ceasing to exist, like the Ogeechee,” Perron says. “And some bigger basins, like the Roanoke and the Savannah, will probably get bigger. All of these boundaries are shifting, and depending on how long you wait, you could see them go very far.”

Kelin Whipple, a professor of earth and space exploration at Arizona State University, says the group’s technique may help scientists understand the architecture of aquifers and petroleum reservoirs, as well as interactions among climate, tectonics, and seismic hazards.

“They have provided a new tool to evaluate the possible influence of these processes and to identify places where they have been active,” says Whipple, who was not involved in the research. “The group’s technique is excellent and will prove a great addition to the arsenal of tools we have to query landscapes about their history.”