Read this story at MIT News
Large river networks — such as those that funnel into the Colorado and Mississippi rivers — may seem to be permanent features of a landscape. In fact, many rivers define political boundaries that have been in place for centuries.
But scientists have long suspected that river networks are not as static as they may appear, and have gathered geologic and biological evidence that suggest many rivers have been “rewired,” shifting and moving across a landscape over millions of years.
Now researchers 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.
The technique focuses on a river network’s drainage divides — ridgelines, such as along mountain ranges, that act as boundaries between two river basins. As rainwater flows down either side of a drainage divide and into opposing rivers, it erodes the underlying rock. The river on one side of a divide may erode faster than the other, creating what the researchers call an “imbalance” in the river network. To reach a balance, they reasoned that a drainage divide must shift to assume a more stable pattern.
The team came up with a measurement technique to determine the direction in which a divide would have to move to bring its river networks into balance, and then made these measurements in actual landscapes, including regions in China, Taiwan, and the southeastern United States. They found that while some river networks matched the stable pattern — suggesting that these are relatively static — other networks, such as those in the southeastern U.S., produced patterns implying that these regions are currently shifting and changing.
Measuring the migration of river networks MIT News video
“We’re able to get a sense of whether a given river network is undergoing dramatic change, and whether individual drainage basins are shrinking, expanding, or migrating laterally,” says co-author Taylor Perron, an assistant professor of geology in MIT’s Department of Earth, Atmospheric and Planetary Sciences. “We take an instantaneous snapshot of the degree of reorganization, and also the direction in which it’s happening.”