A historic logging site shows the first erosion of bedrock caused by man along a river

Geological time is assumed to be slow, and the most solid object should be the bedrock. But a new study from the University of Washington challenges these two concepts: The effects of logging show that human activity can significantly erode the bedrock, resulting in a rapid advance of geology.

The study, published April 15 in the journal Proceedings of the National Academy of Sciences, focuses on the Teanaway River, a scenic river in central Washington State.

"Over the past century, we have more river incisions than expected in this region.Something has caused much greater erosion of these rivers," said Sarah Schanz, Ph.D.'s lead author. University of Washington, Postdoctoral Fellow in Indiana. "We know that the Teanaway River is eroded in the bedrock, of course, some terraces are 1,800 years old, but the current cycle is anthropogenic or human in origin."

The results show that logging practices resulted in a bedrock incision of up to 2 meters (6 feet) along the riverbed. Nearly half of the floodplain has been transformed into a new terrace overlooking the river.

"This is the first time we are able to locate the erosion caused by human activities in the bedrock," said Schanz. "Most rivers erode about one-tenth of a millimeter a year – it's about 100 times more."

The discovery means that this beautiful shore is the result of human acts and not of natural forces. This could change the way geologists think landscapes from other parts of the world, such as Taiwan, with its long history of intense human activity.

The study began 20 years ago when co-author Brian Collins, a senior lecturer in river geology at the University of Washington, was curious about why there was so much exposed bedrock in the Teanaway.

Collins also noticed unusual river terraces, stepped structures along the shoreline resulting from flooding cycles of the river, and then running faster, digging a new canal deeper into the sediments. He led a 2016 study that calculated short-term changes in the western fork of Teanaway and suggested that logging could have caused the river to cut a new channel.

This site in a community forest provided good access for regular visits by the research team and undergraduate assistants to the three ranges. By combining newspaper recordings, UW library special collections materials, Central Washington University and the Kittitas County Historical Society, the researchers were able to reconstruct and confirm the entire story.

Before forest roads existed, companies built temporary "spill dams" high up the slope with all the logs and then broke it with tools or explosives. The liberated water made it possible to send newspapers to the mills.

"It was such an event that schools shut down and newspapers show it very well," said Schanz. "The people who are still alive today, some of their first memories will see him."

The key to the process is that the lumberjacks would remove debris to give the logs a free kick in the river. This removed the barriers that trapped the sediments and removed much of the gravel from the river bed. The authors believe that such events have caused a radical change in erosion.

"If you have too much sediment, you're protecting the river from erosion, but if you do not have enough, as these sediments move, they start to hit the bedrock and get in there." Erode, "said Schanz.

David Montgomery, Professor of Earth and Space Science at UW, and the other two co-authors used many techniques to analyze the four youngest terraces by the river, including maps. LIDAR, carbon dating of rocks and computer models. In 1999, the team even drove nails into the bedrock and measured erosion rates directly.

Many rivers, including the Teanaway River, have individual characteristics that highlight a human impact on the bedrock areas. But this is the first time that human activity has transformed an entire river basin.

"This is a direct topographic signature of the Anthropocene," the age of humans "in which we live," Montgomery said. "The discovery that the terrace surfaces in the Teanaway are recently abandoned floodplains suggests that similar landforms in the world could also reflect the influence of human activity."

The UW team recently published a discussion paper on the state of river terracing over the past 4,000 years. The authors showed that in many cases the formation of river terraces coincided with deforestation.

"It's kind of a hand-wave link, but I think it could be spread around the world," Schanz said. "It's just not a signal we had to look for before."

Schanz will begin teaching at Colorado College in August, where she also plans to explore what it means to discover the formation of river canyons in natural processes.

"I think the human part is really interesting, but what has broader implications, for me, is the evidence that if you change how sediment moves in a river, you can change the rates erosion, "said Schanz.