Humanity hit fast forward on geological time, and there's no rewind

Loggers changed the way a river carved into the earth’s crust.

a river with rocks around it
Rivers usually take thousands of years to cut through bedrock. Loggers sped up that process.Deposit Photos

Without much effort, humans can warm the atmosphere, turn rainforests into deserts, and bleach coral reefs. Now, according to a study published April 15 in the Proceedings of the National Academy of Sciences, we can also literally erode bedrock.

Besides the name of the fictional home of the Flintstones, bedrock is a hard, tightly-packed layer of rock that makes up the bulk of the earth’s crust. A typical cross section of the land beneath our feet might include grass, soil, sand, and gravel, but bedrock provides a solid foundation for all that to sit on. Riverbeds can also be made out of bedrock, though there are certain types of rivers—called alluvial rivers—that have sediment bottoms. When bedrock is exposed to some kind of force, like wind, a moving river, or a glacier, it can slowly and naturally erode over thousands of years, changing the crust’s topography.

Depending on a variety of factors like slope, depth, and sediment, that erosion can wear away between a fraction of a millimeter and a centimeter or two of bedrock each year. But in a tributary east of Seattle, Washington, researchers found that human activity related to logging significantly increased that rate.

Sarah Schanz, a postdoctoral researcher at Indiana University and the study’s lead author, drew on data and historical information to investigate how logging in the late 19th and early 20th Centuries caused increased erosion of the Teanaway River’s bedrock.

Both a river’s steepness and its overall volume can impact erosion rates, but another factor is the amount of sediment present at a river’s bottom. This small, abrasive debris, which includes elements like sand and gravel, acts like a nail file, slowly grating at the bedrock as it drags through the bottom of a river.

This action typically causes a river to erode downward, carving deep valleys into a landscape over thousands of years. But both too much or too little sediment can change the direction of the erosion. Little to no sediment causes a river to erode side to side, creating a floodplain. A high amount of sediment can gather on top of the bedrock and remain stationary, preventing the nail file action.

“It’s sort of like a Goldilocks scenario,” Schanz says.

The amount of debris readily available strongly influences how much sediment rests in rivers. But there’s another factor potentially at play: Obstructions along the river. In the Pacific Northwest especially, Schanz says log jams might have caused many of these obstructions. Log jams are natural pseudo-dams that form when wood from fallen trees or branches makes its way into the river. The blockages control how much sediment flows through the river and, as a result, directs the angle of erosion (side-to-side or downward).

Schanz says she’d expect the Teanaway to naturally erode side-to-side, by about a tenth of a meter every hundred years. But during the 20th century, that number rose to nearly 2 meters as the river cut downward, transforming its floodplain into a steep terrace of rock.

While other natural factors like melting glaciers and fires can free up sediments and increase river erosion, Schanz’s team couldn’t link any of them the Teanaway. There was one culprit left: humans.

People began logging in the Teanaway area around the 1880s, using a variety of practices characteristic of the time. At issue in Schanz’s study was the log drive, a technique loggers used to transport chopped-down trees. They’d block the river with a temporary dam (called a splash dam), creating a temporary reservoir to hold the logs. They’d then blow the splash dam up with dynamite, flooding the river and quickly sending the logs downstream to the lumber mills.

But they had to make a clear path for the logs to follow, and that meant removing the log jams that had naturally formed downstream. This freed up sediment to make incisions in the bedrock at a much faster rate.

“You basically remove the sediment traps that nature had put there, and the river responded,” says David Montgomery, a professor of geomorphology at the University of Washington who also worked on the study.

Log drives were often community affairs—kids would even get the day off school to watch the river flood—and they were well-documented in historical records. Schanz used these to create a timeframe around logging and erosion at the turn of the 20th Century, and the data lined up almost perfectly.

Schanz says logging had a two-fold effect on log jams: The use of log drives removed the natural dams, and the clearing of trees meant there was less available wood to form them again. It was a permanent change for the Teanaway: Montgomery compared the current rate of erosion to about the width of a thumbnail a year—before logging, he says, it would’ve been just the thickness of a thumbnail a year.

“Logging went on for a little over 20 years, and there’s still this long lasting effect,” Schanz says.

While humans can physically take jackhammers and excavators to bedrock, Montgomery says there hasn’t been much thought about how we can unknowingly influence a geological process that happens over thousands of years.

“We’re literally changing the surface of the earth in ways that you can quantify and measure, and that will persist,” he says.

Montgomery says the research demonstrates that humans are still uncovering the ways in which our activities can have profound effects on our environments.

“We’re doing a pretty effective job of remodeling the planet that we live on, and yet we’re still discovering elements of how it works,” he says. “How many people want to remodel their house without a blueprint or an architect or an understanding of what’s behind the walls?”

Schanz’s research shows us that we have yet to understand the full impact of the age of the anthropocene, a term scientists coined to describe an era when intense human activity is the predominant influence on many of the earth’s natural processes. In just 20 years, humans were able to speed up a geological process that would typically take thousands—and all that eroded bedrock is gone for good.

Note: A previous version of this article stated that the Teanaway River is west of Seattle when, in fact, it is east. We regret the error.