Erosion hits the accelerator (Image: Kristen Cook)

Blink and this gorge will be gone. Normally, erosion takes thousands of years, but this river valley could vanish just 50 years after it formed, thanks to a new and rapid process called “downstream sweep erosion”.

“In terms of river erosion, the Daan river gorge in Taiwan is extremely rare,” says Kristen Cook of the German Research Centre for Geosciences in Potsdam.

The gorge formed after an earthquake in 1999 that lifted the ground over a distance of a kilometre, blocking a river. By 2004, the river had broken through a chink in the blockage. By 2008, it had gouged out the kilometre-long gorge, which is 25 metres wide and reaches depths of 17 metres.


But it just so happens that the river has to bend through 90 degrees before entering the gorge. That means it flows at right angles to the line of the gorge just above the gorge mouth. This sideways flow makes the river extremely abrasive. In effect, it acts like a sheet of sandpaper, grinding away the upstream wall of the gorge at 17 metres a year. At this rate, the whole gorge will vanish in about 50 years.

“As the upstream boundary of the gorge keeps moving downstream, the gorge will get shorter and shorter until the upstream boundary reaches the exit of the gorge, and the whole thing is gone,” says Cook. All that will be left will be a broad, flat floodplain, just like before the earthquake.

The speed of the erosion is unusual, because geology is normally slow. A few geological events do happen quickly, such as islands forming after volcanic eruptions, landslides and floods from glaciers. But while the erosion of a gorge would normally take centuries, in Daan it will happen within a human lifetime. “It lets us monitor and study it in great detail,” says Cook.

Journal reference: Nature Geoscience, DOI: 10.1038/NGEO2224