Rare case of 'river piracy'

Photo: The Canadian Press

Scientists have witnessed the first modern case of what they call "river piracy" – and they blame global warming. Most of the water gushing from a large glacier in northwest Canada last year suddenly switched from one river to another.

That changed the Slims River from a three-metre deep, raging river to something so shallow that it barely was above a scientist's high top sneakers at midstream. The melt from the Yukon's Kaskawulsh glacier now flows mostly into the Alsek River and ends up in the Pacific Ocean instead of the Arctic's Bering Sea.

It seemed to all happen in about one day — last May 26 — based on river gauge data, said Dan Shugar, a University of Washington Tacoma professor who studies how land changes. A 30-metre tall canyon formed at the end of the glacier, rerouting the melting water, Shugar and his colleagues wrote in a study published in Monday's journal Nature Geoscience.

The term "river piracy" is usually used to describe events that take a long time to occur, such as tens of thousands of years, and had not been seen in modern times, especially not this quickly, said study co-author Jim Best of the University of Illinois. It's different from something like the Mississippi River changing course at its delta and it involves more than one river and occurs at the beginning of a waterway, not the end.

The scientists had been to the edge of the Kaskawulsh glacier in 2013. Then the Slims River was "swift, cold and deep" and flowing fast enough that it could be dangerous to wade through, Shugar said. They returned last year to find the river shallow and as still as a lake, while the Alsek, was deeper and flowing faster.

"We were really surprised when we got there and there was basically no water in the river," Shugar said of the Slims. "We could walk across it and we wouldn't get our shirts wet. It was like a snake-shaped lake rather than a river."

The Kaskawulsh glacier covers about 25,000 square kilometres, about the size of Vermont. The front of the glacier has retreated nearly 1.9 kilometres since 1899, Shugar said.