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An image taken during field work in the Daan River gorge, Feb. 8, 2010. The large outcrop in the center of the photo disappeared in the space of an hour during a flood in 2012. (Kristen Cook, GFZ)

This Gorge Is Living Its Life on Fast-Forward

A quickly carved river gorge may disappear in just a few decades

smithsonian.com

Erosion generally isn’t a particularly quick force of nature. It can take millions of years to level mountains into hills or carve canyons into the earth’s surface. But in one place in Taiwan, geologists have had the chance to witness the surprisingly quick formation and ongoing destruction of a river gorge, which is leading to new insights on a very old force of nature.

In a new paper in Nature Geoscience, a team of researchers from GFZ German Research Centre for Geosciences in Potsdam describe a new kind of erosion that they’ve been able to witness first-hand in a gorge in Taiwan. 

In 1999, a huge earthquake lifted up part of the earth's crust by 32 feet, creating a natural dam across the Da'an Chi River where it runs through a valley. "The amount of uplift was huge," the study's lead author, Kristen Cook, told the BBC. "Imagine one side of your house going up by 10m - it's a big change." 

Then, in 2004, that dam got overrun by the river, which started eating into the soft bedrock of the valley at an unusually fast pace, creating a steep gorge. The gorge itself stretched for 3,280 feet, and it was 82 feet wide and 55 feet deep. 

 “Before the quake there was no sign of a gorge at all in this riverbed, which is one and a half kilometers wide," Cook explained in a press release, “We have here the world’s first real-time observation of the evolution of gorge width by fluvial erosion over the course of several years.”      

But now, just a few years after the gorge was formed, the river isn't just digging into the same kind of soft bedrock. It's now digging into the rougher sediments that built up behind that dam in the five years after the earthquake, and it's carrying them downstream in huge, typhoon-induced floods.

This last development is basically wiping out the gorge that was just built. The rough sediments from upstream crash into the softer walls of the gorge, eating away at those steep walls and making the river channel wider. The authors call this "downstream sweep erosion" and estimate that it is destroying the gorge at a rate of 55 feet each year.

If the rate of erosion keeps up, in 50 or 100 years, there won't be a gorge anymore, the scientists think. It's just going to be another river valley, just like it was in 1999, before the earthquake. and there won't be any trace of this landform left. 

Usually, geologists have assumed that this kind of process takes thousands or millions of years.  The Grand Canyon, which was carved by more staid erosive processes, took millions of years to get to its current state (though there is still some debate as to whether it was 6 million years or 70 million). And as a very rough point of comparison, Niagara Falls (which is much larger and has different rocks underlying it), is estimated to have another 50,000 years left before it disappears.

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