High resolution snapshot of a forest dieoff caused by a logjam in 2009 along the Cubrene River in the Bolivian rainforest. Image: Google Earth

In 1996, a tree fell in the Bolivian Amazon, jamming up a river like a chicken bone lodged in a drain. 119,000 acres of forest died in the catastrophic flooding that ensued. Twenty years later, new research has concluded that this wasn’t a one-off event: So-called ‘logjams’ are constantly reshaping vast swaths of western Amazonia.

That’s according to Umberto Lombardo, a geographer and geomorphologist at the Pompeu Fabra University in Barcelona, Spain, who first became interested in how logjams were affecting landscapes several years back. He was playing around with Google Earth timelapse, and noticed certain Amazonian rivers behaving “in a very peculiar way.”

“I became very curious,” he told Earther.

That curiosity led Lombardo to hunt for papers describing what could be causing rivers to clog up suddenly, spilling their contents into the surrounding forest. All he could find was a study from 1996 describing a logjam in the Cuberene River in Bolivia that caused a large area of forest to die off and regenerate as savanna, a grassland peppered with trees.

Another study by the same author explored how satellite imagery can be used to identify logjams, but Lombardo could find no follow-up research detailing how prevalent these natural events actually are in the tropics.

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To find out, Lombardo downloaded Landsat satellite imagery of a 9,300-square-mile swath of the Bolivian rainforest, captured from 1984 to 2016. He identified 22 rivers that showed signs of logjam-induced flooding, and analyzed eight of them in detail, tallying up the total number of logjams and exploring how these events impacted the surrounding forest.

Evolution of the Cuberene River over time as successive logjams caused downstream flooding and converted forest to savannah. Image: Lombardo 2017

The results, published recently in Earth System Dynamics, showed that logjam induced floods are “a major driver of forest disturbance in the Bolivian Amazon,” affecting hundreds of acres of forest, often on an annual basis. Logjams affect rivers in characteristic ways, drying up the downstream channel and diverting water into the forest in a v-shaped area.

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The forest then drowns, and is often replaced by savanna. In one of the areas Lombardo studied, the Cuberene river, the 46,000 acres of forest disturbance caused by logjams was “comparable to the current rates of deforestation driven by the expanding agricultural frontier.”

This is just a single exploratory study, of course. Much more research is needed to suss out how prevalent logjams are throughout the tropics, and how the ever-increasing human footprint in rainforests, including deforestation, affects these natural events.

Sand deposited overnight near a river in which a logjam formed. Image: Lombardo 2017

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In the Bolivian Amazon at least, Lombardo thinks logjams help shape forest composition, and drive regeneration. When you’re out in the rainforest, he says, the effects of these river clogs aren’t subtle.

“It was impressive,” he said, describing a visit to a forest that had been flooded by a logjam. “The ground was covered with fluvial sediments, the trees had no leaves because they were all dead, and the sun’s rays reached the earth. It looked like a forest that had been burned down but without ashes or charcoal.”

And you thought a single tree couldn’t make a difference.