Its water rerouted by a retreating glacier, the Slims River offers researchers an extreme example of ‘river piracy’ – one with far-reaching implications for northern waters, Ivan Semeniuk explains

A stream flows through the toe of Kaskawulsh Glacier in Yukon’s Kluane National Park. In 2016, this channel allowed the glacier’s meltwater to drain in a different direction than normal, resulting in the Slims River’s water being rerouted to a different river system.

Daniel Shugar knew his research trip was in trouble when he and a colleague arrived at Kluane Lake, the largest body of fresh water in Yukon, last August.

A Canadian geomorphologist based at the University of Washington in Tacoma, Dr. Shugar’s plan had been to measure water currents at the mouth of the Slims River, which normally spills down from the mountains of Kluane National Park and feeds the lake from its southern end.

There was only one problem. Except for a lingering trickle, the river was gone.

“We were pretty shocked,” said Dr. Shugar, who already had a sense that something was up based on local news reports. “But we had no idea what was really in store.”

At that point, an expedition to study the river turned into a quest to understand why it had effectively vanished. One chartered helicopter ride later, Dr. Shugar had an answer – which, together with a team of U.S. and Canadian researchers, he has since backed up with a careful analysis.

According to their report, published Monday in the journal Nature Geoscience, the cause of the unprecedented disappearance can be unequivocally chalked up to climate change.

For years, the warming atmosphere has caused the toe of the Kaskawulsh Glacier, one of Canada’s largest, to retreat across the high mountain valley that is the source of the Slims River. Then, last year, pooling meltwater finally managed to cut across the thinning glacier, opening a new outlet.

Instead of draining northward down the Slims toward Kluane Lake, the Yukon River and, eventually, the Bering Sea, the water abruptly began flowing in the opposite direction. That took it into the Kaskawulsh River, a tributary of the Alsek, which runs southward to the Pacific. Following this route, the former headwaters of the Slims River now reach the ocean some 1,330 kilometres away from where they would otherwise have ended up.

The case is a first for the scientific record books, an extreme example of what geographers call “river piracy,” when the drainage of one watershed is stolen by another. But on this occasion the shift occurred virtually overnight.

“To me it’s kind of a metaphor for what can happen with sudden change induced by climate,” said John Clague, who holds a chair in natural hazard research at Simon Fraser University and was a co-author in the study.

While people may inherently think of climate change as a gradual process, its effects need not be, he said, adding, “I think that has important implications for society.”

Signs of the rerouting have been observed on both sides of the mountainous divide. Gauges on the Alsek River reveal that it experienced a record discharge last year. Because the river mostly flows through parks and protected lands, the increase has had no immediate human impact.

On the Slims side, the effect of water loss is more obvious. Last summer, Kluane Lake dropped a full metre below its lowest ever recorded level for that time of year. The reduced inflow from the Slims River spells a huge change for the 65-kilometre-long lake, with implications for nearby communities and visitors who access its waters for fishing and other activities.

“This is likely to be permanent,” said Dr. Shugar who believes the warming trend that caused the Kaskawulsh Glacier to thin so dramatically all but ensures the change is irreversible for the foreseeable future. “If that’s the case, Kluane Lake is only going to get smaller.”

The Kaskawulsh River, as it exits the lower terminus of Kaskawulsh Glacier and lakes. ‘River piracy’ gave the Kaskawulsh the waters of the Slims River waters in 2016.

The conclusion relies on a recently developed computer model that shows it is essentially impossible that the glacier could have receded so much, so quickly without the influence of fossil-fuel emissions causing Earth’s atmosphere to warm.

Garry Clarke, a glaciologist at the University of British Columbia who was not involved in the study, said the team’s attribution of the pirating of the Slims River to human-caused climate change is “almost certain to be correct.”

The team also suggests the piracy isn’t over. With the glacier no longer a barrier, there is nothing to stop the Kaskawulsh River from nibbling further and further up the course of the Slims River Valley to capture more of its side tributaries. Eventually, perhaps many decades from now, Kluane Lake itself may begin draining south toward the Pacific, a suggestion that Dr. Clarke said he found “extremely intriguing and entirely plausible.”

“So, we are not done with this,” he added.

A twist to the story is that such a scenario would return the lake to a configuration that likely existed for thousands of years, until approximately 1700, when a cooling trend allowed the Kaskawulsh Glacier to advance and divide water flow in the valley.

Now that the glacial barrier has been breached, the long-term impacts for ecosystems on either side of the former divide has yet to be determined, said Dr. Shugar, who noted that there were no biologists on the team.

“It’s a unique situation,” he said. “In the end, it’s turned into a superb opportunity to study some of the underappreciated consequences of climate change.”

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