A shaded relief map of part of the Dan River basin, in Virginia, shows lower elevations in blue and higher elevations in red. The map is the result of technology being developed by researchers at the Massachusetts Institute of Technology and the Swiss Federal Institute of Technology that tracks the movement of ridgelines and their associated river channels over millions of years. Scott McCoy, MIT
A measurement technique developed by researchers in Massachusetts and Switzerland makes it possible to map a river network’s past migration and future movement.
March 18, 2014—Tectonic uplift creates mountain ranges, but it is the movement of water down drainage slopes that creates river channels and erodes that uplifted rock. Now researchers based at the Massachusetts Institute of Technology (MIT) and the Swiss Federal Institute of Technology, in Zurich, have for the first time created a measurement technique that makes it possible to map the movement of ridgelines and their associated river channels over millions of years—taking into account tectonic uplift and erosion—to predict a river channel’s direction of movement in the future.
Knowing how river channels have moved, and predicting how they will move, impacts how humans interact with the natural and built environments. “We live on landscapes, we engineer the topography,” says Taylor Perron, Ph.D., an assistant professor of geology in the Department of Earth, Atmospheric, and Planetary Sciences at MIT. “We rely on river networks for a variety of things, including water resources, agriculture, the transport of sediment and nutrients.…Knowing what controls the structure of river networks, and how the landscape that we live on and exploit has been constructed, is important.
“If you look at a conventional map of a river network, it is not clear at all how that river network is changing,” says Perron. “Our mapping technique allows us to estimate the direction that the drainage divides that separate river basins are moving. And if we can estimate that, then we can estimate how the shapes and sizes and locations of all those drainage basins are changing through time.”
The motion of a ridgeline that forms a drainage divide depends on the water-induced erosion occurring within the drainage basins on either side, Perron explains. “If one side of a divide is eroding faster—[for example] if the river on that side of the divide is cutting down into the bedrock faster than on the other side—then the drainage divide is going to migrate away from the faster-eroding river,” he says. “The problem is that it’s quite difficult to measure those erosion rates because they are really, really slow. Typically a fraction of a millimeter a year.”
The calculation technique relies on a proxy, referred to as , that predicts a river’s elevation under a scenario in which erosion balances tectonic uplift. By knowing the value of that hypothetical state of equilibrium, Perron and his colleagues could then analyze how the actual river channels located in drainage basins on either side of the same ridgeline differ from that perfectly balanced state. Over time, “the divide is going to move in the direction of the river that has the higher equilibrium elevation,” Perron says. The movement of divides between two river channels causes basins to grow or shrink over the span of millions of years.
This technique can be used anywhere there are rivers cutting through bedrock. “It works best if you do this in places that have pretty uniform rock type and climate, or at least where the spatial patterns in rock type or climate are consistent among the river basins you’re comparing,” Perron notes. “It’s possible to incorporate differences—tectonics, climate, rock type—but its easier if those are consistent.”
The initial paper, “Dynamic Reorganization of River Basins,” was published in the March 2014 issue of Science. The paper presented the new mapping technique and examined three landscapes: the Loess Plateau in China, where fine, wind-blown sediments have accumulated over the past 2.6 million years to create thicknesses of up to hundreds of meters; a section of Taiwan’s eastern Central Range, where the island itself was formed by an ongoing collision of the Luzon volcanic arc with the Eurasian plate that began a few million years ago; and the region surrounding the Blue Ridge Escarpment in the southeastern United States, which divides the coastal Piedmont plateau and the Appalachian mountains.
These three landscapes were selected because they span a range of characteristics that are expected to influence the extent of river basin reorganization, according to Perron.
The Loess Plateau “has been fairly tectonically stable in the recent geologic past, and also—at least right at the surface—is made of material that is very easily eroded,” Perron explains. “In that landscape we expected to see a river network that was more stable, where the drainage divides were not moving a lot—and in fact that is what we saw,” he says.
The eastern central range of Taiwan provided “a young, active mountain range that is uplifting and eroding really fast,” Perron says. “That’s a place where we expected to see a lot of dynamic adjustment of the river basins, and we do.”
The final landscape, the Southern Appalachians in the southeastern U.S., between the mountain’s crest and the coast, was chosen because the researchers expected it to be a very quiet area, Perron says. “This is a landscape that has not experienced a lot of tectonic activity for hundreds of millions of years,” he says. But surprisingly, the researchers discovered that the river basins within the mountains were still changing quite dramatically.
“So the upshot from this, and the thing that surprised us most, is that earth’s surface topography is a lot more dynamic than we thought it was. Now granted, this is happening over geologic time, so by dynamic I’m talking millions of years,” Perron notes. “But still, in a landscape that is what most people would describe as a dying mountain range, the river basins are still changing a lot.”
In the future, the calculation technique developed by the researchers could form the basis of a more complicated model that could predict how climate change and precipitation patterns could impact specific river channels. “It’s doable; it’s just not something that we’ve done yet,” Perron says.
Currently the researchers are examining which forces determine whether a drainage divide migrates gradually across a landscape or is abruptly rewired, according to Perron.
In addition to Perron, four authors contributed to the development of the technique and the paper’s findings: Sean D. Willett, Ph.D., a professor at the Geological Institute of the Swiss Federal Institute of Technology (ETH) in Zurich, Switzerland; Scott W. McCoy, Ph.D., previously a joint postdoctoral fellow at MIT and the Swiss Federal Institute of Technology and now an assistant professor in the Department of Geological Sciences and Engineering at the University of Nevada, Reno; Liran Goren, Ph.D., previously a senior scientist at ETH and currently a senior lecturer in the Department of Geological and Environmental Sciences at Ben-Gurion University of the Negev in Beer-Sheva, Israel; and Chia-Yu Chen, a doctoral student at ETH.