February 21, 2018. Heavy rainfall led to a rapid flow that entrained rocks of considerable size in New Zealand. Observer Donna Field was alongside the Rakaia River in Canterbury and captured pictures and video of the event.

Canterbury’s aptly named Terrible Gully turned to river of rock by ex-Cyclone Gita

Professor of Geophysics at Victoria University Tim Stern saw the video and thought it was an impressive sight.

“You wouldn’t want to be trapped by it – it’s a scary example of how vulnerable our environment is to be destroyed by weather,” he said.

New Zealand has one of the highest erosion rates in the world, alongside the eastern Himalayan syntaxis, he said.

“This exemplifies that and shows the effect heavy rainfall has on New Zealand’s weak greywacke basement rocks, which are much weaker than other basement rocks, like in Australia.”

Monsoon rains, in combination with rocks pushed up very quickly by tectonic uplift, means they are vulnerable to erosion.

“The rain is what weakens them and causes it to erode very quickly, it’s why we have one of the highest erosion rates in the world.”

This kind of impressive flow happens repeatedly and cuts off farmers from a nearby town. See video here (embedding is blocked)

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