What caused a Canadian river to disappear almost overnight?

Posted 21 April 2017

A Canadian river has vanished in just four days, becoming the first known case of 'river piracy' in modern times.

The climate-driven retreat of one of Canada’s largest glaciers – the Kaskawulsh Glacier – has abruptly and radically altered the regional drainage pattern in spring of last year, a study has documented.

“Geologists have seen river piracy, but nobody to our knowledge has documented it happening in our lifetimes,” said lead study author and University of Washington Tacoma Geologist Dan Shugar.

“People had looked at the geological record – thousands or millions of years ago – not the 21st century, where it’s happening under our noses.”

River piracy, also known as river capture, takes place when a river drainage system or watershed is diverted from its own bed and flows to a neighbouring stream bed instead.

Kaskawulsh Glacier meltwater has drained into the Slims River before being carried north to the Bering Sea for hundreds of years.  

But last year intense melting meant the drainage gradient was tipped and the water changed course, flowing thousands of kilometres in the other direction after joining the Kaskawulsh River and heading south toward the Gulf of Alaska. 

River piracy can happen for a range of reasons, including tectonic motion of the earth’s crust, landslides and erosion. 

In this case, a UW technique shows a 99.5% probability that this glacier’s retreat was a result of modern climate change.

“Based on satellite image analysis and a signal-to-noise ratio as a metric of glacier retreat, we conclude that this instance of river piracy was due to post-industrial climate change,” the report stated.

“Rapid regional drainage reorganisations of this type can have profound downstream impacts on ecosystems, sediment and carbon budgets, and downstream communities that rely on a stable and sustained discharge.”

The occurrence came as a surprise to Shugar and his colleagues, who had been planning fieldwork on the Slims River last year, only to arrive in August to discover the river was not flowing

Looking back over river gauge data, they identified an abrupt drop over four days from May 26 to 29, 2016.

The research team used a combination of hydrological measurements and drone-generated digital elevation models to map how the river changed direction.

“What we found was the glacial lake that fed Slims River had actually changed its outlet … a 30m canyon had been carved through the terminus of the glacier,” Shugar said.

“Meltwater was flowing through that canyon from one lake into another glacial lake, almost like when you see champagne poured into glasses that are stacked in a pyramid.”