The locations of drainage divides determine how water flows across a landscape. Now, a new study has revealed how quickly these features can migrate when a region’s normally dry climate gets a little wetter.
Researchers used a combination of field observations, sediment dating, and numerical modeling to show how a river system in Israel’s Negev Desert has made sudden shifts in response to known wet periods. Drainage divides that migrated at an average rate of 1.1 kilometers (0.7 mile) per million years over the studied interval stalled and picked up speed in step with known shifts in the region’s climate.
“To my knowledge, this is the first study that directly measures rates of drainage divide migration,” said Mikaël Attal, a geomorphologist at the University of Edinburgh in Scotland who was not involved in the research. “This is important because drainage migration can have implications for understanding erosion across landscapes, our ability to infer tectonics from topography, and the management of water resources.”
Drainages Move Slowly, Then All at Once
Water falling on a landscape flows downhill, accumulates in rivers, and eventually drains into lakes, wetlands, or oceans. Drainage divides are topographic boundaries that control the water’s path.
“If a drop of water falls on one side or the other of a drainage divide, it will follow a different route,” Attal said. He used North America’s Great Divide as an example: “If a drop falls on the west side of the divide, it goes to the Pacific; if it falls on the other side, it goes to the Atlantic.”
A drainage divide migrates when the rivers on one side of the ridge erode more rapidly than on the other. In response, rivers may change their course or even reverse direction.
Because divide migration has a significant effect on landscapes, researchers are interested in how—and how quickly—it happens. But so far, it has been difficult to determine the rate at which drainage divides migrate on short timescales.
“Geomorphic markers capable of recording past divide locations, such as alluvial terraces, are often eroded away,” explained Elhanan Harel, a geomorphologist at the Geological Survey of Israel and a coauthor of the study, which appeared in Proceedings of the National Academy of Sciences of the United States of America.
Most of the movement happened in two intervals, during which the drainage divide migrated across the landscape at twice the average rate.
Previous studies have used cosmogenic nuclide dating to measure erosion on either side of drainage divides. These erosion rates, along with other variables, were fed into an equation that estimates the rate of drainage divide migration.
This approach, however, presents several drawbacks. The equation relies on a simplified geometric model of the drainage divide, which may not accurately describe a specific site. In addition, cosmogenic nuclide dating provides erosion rates that are averaged over a river basin, which may not match erosion at the drainage divide. The erosion rates inferred from cosmogenic nuclides are also time averaged, making it impossible to track short-term changes.
Harel and his coauthors overcame these difficulties by studying unusually well-preserved river terraces in Israel’s dry southern Negev Desert. River terraces, created as rivers slowly erode and leave behind steps that represent previous levels of the valley floor, are valuable markers of regional geomorphology. At the Negev Desert site, each terrace records a past location of the drainage divide, enabling Harel and others to trace the divide’s migration step by step.
The researchers used a technique called optically stimulated luminescence to date when the terraces formed. Collating dates on the sequence of terraces, they reconstructed the drainage divide’s 258-meter migration over the past 227,000 years.
Most of the movement, they found, happened during two intervals, from 245,000 to 183,000 years ago and 36,000 to 26,000 years ago, during which the divide moved across the landscape at twice the average rate.
Wet Climates May Drive Rapid Migration
Although the southern Negev Desert has been mostly dry for at least a million years, its arid state has been punctuated by occasional wet periods: One occurred around 220,000–190,000 years ago, and another took place between 35,000 and 20,000 years ago. These periods coincide with intervals of rapid drainage migration.
Increased weathering and the timing of groundwater recharge in the southern Negev indicate that extreme storms and floods occurred at those times.
The researchers simulated the physical processes of river incision to evaluate whether climate shifts could explain the observed divide migration rates. They found that a scenario assuming constant climate conditions couldn’t reproduce their observations from the Negev, but one that included intermittent climate shifts matched the results exactly.
“Our study provides the first direct evidence linking divide migration to climate fluctuations on much shorter timescales.”
The new analysis supports the idea that climate and rainfall drive landscape changes. “While previous studies have demonstrated that tectonic forces can drive divide migration over million-year timescales, our study provides the first direct evidence linking divide migration to climate fluctuations on much shorter timescales,” Harel said.
Attal agreed that the study helps researchers understand the connection between climate and drainage patterns. “It is very interesting that [the authors] found that the divide tends to migrate in bursts during wet periods,” he said.
This knowledge may be increasingly relevant as extreme weather events—such as severe rain, storms, and floods—become more common because of climate change. In flat areas with an abundance of loose sediment, severe flooding could divert rivers and shift drainage divides, causing permanent changes to the landscape.
“I think this work highlights that some landscapes may be highly sensitive to climate change,” said Attal.
—Caroline Hasler (@carbonbasedcary), Science Writer
