A story written in mud
Geologists have long assumed that the evolution of land plants enabled rivers to form snakelike meanders, but a review of recent research overturns that classic theory – and it calls for a reinterpretation of the rock record.
Meandering rivers twist, bend and swirl across the landscape, marking the globe like paintbrush strokes on a planet-sized canvas. Their winding channels have supported human settlements since their dawn and shaped the history of our civilization.
These dynamic streams have also facilitated the formation of flood plains and the deposit of critical minerals for millions of years. Based on analyses of the rock record, geologists have long assumed their snakelike bends were only made possible by the evolution of plants that help stabilize riverbanks. As it turns out, it’s not that simple.
“What was previously interpreted as the appearance of meandering rivers triggered by land plant evolution is actually just a change in the way that meandering rivers and their deposits get recorded into rocks,” said geologist Mathieu Lapôtre, co-author of a paper published Jan. 25 in Nature Reviews Earth & Environment. “A new picture is now arising from all of this work, which is that meandering rivers certainly existed before plants. And that is a very important realization for the way we think about Earth’s surface environments and climate through time.”
What rivers tell us
By studying how rivers behave in the modern and ancient world, researchers may be able to understand their role in our changing planet. For example, what happens when fires destroy the vegetation on the banks of a meandering river? Will it turn into a braided river with numerous strands and deliver a destructive pulse of sediments downstream? How will dams affect our river systems and their ecology on a long-term scale? How much organic matter could have accumulated in flood plain sediments without meandering rivers?
“We tend to assume that geological processes have remained uniform through time,” said Lapôtre, an assistant professor of geological sciences and, by courtesy, of geophysics at Stanford’s School of Earth, Energy & Environmental Sciences (Stanford Earth). “It’s hard for us to imagine all of the factors that could have been different in the past, but ultimately, over geologic timescales, only the laws of physics remain constant.”
For decades, scientists have assumed that virtually all rivers on Earth prior to the evolution of rooted land plants about 420 million years ago during the Devonian Period were braided, not meandering rivers.
“There’s a new recognition that there’s actually a different way of organizing all this, and it applies to the first 90 percent of Earth’s history – that’s really important to know,” added co-author Kevin Boyce, a professor of geological sciences at Stanford Earth.
A new perspective
Meandering rivers are single channels that wind back and forth like snakes, often developing on low-gradient, fertile plains. On the other hand, braided rivers consist of numerous channel strands separated by small sediment islands. They often occur in steeper slopes and with variation in the amount of water they carry.
Compared with their braided counterparts, meandering rivers tend to be relatively narrower, deeper and more stable, offering suitable sites for centuries of human development – and an important control on carbon cycling.
Meandering rivers are dynamic hydraulic systems that eventually emerge into a lake or ocean. As sediment moves through meandering rivers, the bends and curves affect where, how often and for how long particulate carbon is stored, and that process can impact the short-term carbon cycle that affects human sustainability. Meandering rivers may even affect the long-term carbon cycle, which relies on carbon sinks and the weathering of rocks to balance the gases released from erosion and major volcanic eruptions, according to Boyce.
Looking to other planets
The geological process of river formation is not unique to our planet. Scientists, enraptured by otherworldly swirls, study the physics of their formation to reveal clues about the mysterious art of ancient Earth.
When Lapôtre started exploring the idea of vegetationless meandering rivers, he turned to a neighboring planet for a grand-scale experiment: Mars.
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Known to be void of land plants throughout its 4.5 billion-year history, the Red Planet offers a glimpse into what may have chiseled the landscape of early Earth. Lapôtre and collaborator Alessandro Ielpi of Laurentian University, lead author of the review paper, found the answer in a material that’s not quite rock and not quite sand.
“We see the remnants of old meandering rivers on Mars – it may not have had plants, but it had a lot of mud,” Lapôtre said. “Seeing that was a major clue that led us to explore whether mud could enable the formation of river meanders.’”
Mud is a material that can elude the rock record – or in this case, cause a reinterpretation of it. In order to identify a meandering river, geologists have historically looked for inclined layers of alternating sand and mud. Since they couldn’t find that pattern in rock deposits older than plants, they assumed meandering rivers didn’t exist then. But based on fieldwork in remote meandering rivers without vegetation, Lapôtre and Ielpi found the deposits were very different: Instead of exposing alternations of sand and mud, they showed that everything was mixed together.
“We don’t know how to recognize meandering rivers in rocks that were formed in a world without vegetation,” Lapôtre said. “This is a very fundamental realization that we’re having that bears significance to our understanding of Earth’s past, Earth’s future and also other planets and early surface environments.”
When they further explored the development of meandering rivers, the scientists found that without vegetation, river meanders sweep across the landscape up to 10 times faster than over land with plants.
“Before vegetation evolved, organic matter would have spent a shorter time in the flood plains and that affects the short-term carbon cycle because the rates of the biochemical cycle will be pretty different,” Lapôtre said.
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Tools for the future
While this new interpretation is exciting, the researchers now realize they lack the techniques necessary to identify meandering rivers devoid of plants in the rock record. Lapôtre’s lab group is on a mission to change that.
“Going back and reclassifying things requires the development of new tools to read that rock record,” Lapôtre said.
The findings demonstrate the value of combining the perspectives of multidisciplinary researchers – and taking the time to process multiple data sources in the format of a review paper. With its implications for climate change and urban planning, the co-authors hope researchers in other fields will translate the findings into concrete results.
“The currency for science isn’t data, it’s insight, and in this case, a review paper that makes new connections is far more valuable than another data table,” Boyce said. “Sediments are our knowledge of Earth, and this is about a real change in sediments – we had no way to think about that, and this paper provides it.”
Martin Gibling of Dalhousie University, a champion of the classic hypothesis that meandering was enabled by land plant evolution, is also a co-author on the paper.
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