In 4 days, a river that had flowed for millennia disappeared

Nearly a year ago, scientists noticed that the water level of the Slims River in British Columbia was extremely low. So they hopped into a helicopter and flew upstream to investigate. What they found startled them: A second, more powerful river, the Kaskawulsh, had stolen the Slims River’s water for itself. Three days later, the Slims was gone entirely.

For centuries, meltwater from the Kaskawulsh Glacier had fed both the Slims and Kaskawulsh rivers. But in the past 100 years, the glacier has retreated as a result of climate change. And in early 2016, the glacier receded so much that the Kaskawulsh River, with a gradient nearly five times steeper than the Slims, was perfectly poised to catch and divert the Slims.

In a recent paper, scientists say this was most aggressive instance of “river piracy” — one river capturing and diverting the flow of another — on record. It’s also a troubling example of how swiftly climate change is affecting rivers and other bodies of water.

“Most people think of climate change as gradual and its consequences as gradual, but one of the things we were able to show here is you can produce some rather dramatic changes, suddenly,” said John Clague at Simon Fraser University in British Columbia, one of the authors of the paper. “And these unforeseen, sudden changes are much harder to deal with than ones that play out slowly.” In particular, researchers like Clague are worried about sudden changes to the chemistry and nutrient supply of a lake once fed by the Slims.

To understand what this looked like, let’s start with a satellite image of the Kaskawulsh Glacier and the Slims and Kaskawulsh rivers.

Daniel Shugar / University of Washington

As you can see in this aerial shot from May 2016 (after the Slims was pirated), the Slims is a fraction of its former self — largely silt and mud.

Lance Goodwin / Arctic Institute of North America

The water level at Lake Kluane, where the Slims once flowed, has also plummeted at least 3 feet. And massive daily dust storms have torn along the Slims’s dry riverbed, dumping tons of sediment (the gray plumes below) into the lake.

Lance Goodwin / Arctic Institute of North America

As for the Kaskawulsh, its water level has risen dramatically, which has rippled down to rivers it feeds. For instance, the Alsek river (that is fed by the Kaskawulsh and empties into the north Pacific Ocean) was once comparable in size to the Slims river, but experienced peak discharge 81 percent higher than normal in July, according to scientists’ flow measurements.

Jim Best/University of Illinois

Clague cautioned me it’s too soon to know what the lasting consequences of all of this will be. But what alarms him is that this happened so abruptly. Normally, it takes millennia for something like this to happen — not a matter of days.

Melting glaciers aren’t the only thing re-routing rivers. The rapid decline in river ice is also permanently changing Arctic water flow.

It’s not just glacial retreat that’s permanently altering the courses of rivers.

Researchers from the University of North Carolina and Michigan State University found that river icing, or ice that forms from groundwater flowing along a river bed during winter in the Arctic, are rapidly decreasing in size as well as melting sooner each summer.

Arctic river icings are massive, ranging anywhere from 6 miles in area to more than 32 feet thick and are defining features of many rivers in the Arctic.

“These features basically determine the shape of rivers, because they tend to not just form within the river, but over the banks of the river,” said Tamlin Pavelsky, a professor in geological sciences at the University of North Carolina Chapel Hill. “They tend to result in water flowing outside of the normal channels, creating large multi-channel systems and can even form a barrier to water [from glacier melts] moving down a river.”

Here’s an example of what a river icing looks like:

Jay Zarnetske / Michigan State University

“These riving icings develop during the winter and stick around after the snow has melted all around them,” said Pavelsky. “But what we found is that these [icings] are now disappearing significantly earlier — faster than they were even a decade ago.”

River icings usually melt in mid-July, but over the past 15 years, researchers found that they were melting on average 26 days earlier. To show just how much river icing has retreated over the years, Pavelsky sent me two satellite images of river icing along the Hulahula river in the Arctic National Wildlife Refuge.

Here’s the one from 2001:

USGS DigitalGlobe

And here’s the one from 2009:

USGS DigitalGlobe

As you can see, the river icing surface area shrunk dramatically by 2009. Researchers have not yet determined if river icings are shrinking because of warming temperatures or because of how climate change alters river and groundwater interaction.

“As temperatures warm up and there is more permafrost thawing, there is more connectivity between groundwater and river channels and thus more flow during the wintertime,” said Pavelsky. “This might be what’s inhibiting the formation of these icing features and stopping them from freezing, but there’s no evidence of this yet.”

But one thing is certain: Shrinking river icings are already impacting Arctic river ecology and the wildlife that depend on them.

River icings create wide multi-channel riverbeds that serve as important habitats for animals and fish in the Arctic. Pavelsky told me that you can often spot caribou on top of the ice in the summer to escape from mosquitoes. Even more crucially, they’re an important source of fresh water for wildlife as there’s not a lot of rainfall in the Arctic during the summer. But as river icings continue to melt sooner and more rivers suddenly dry up — much of the Arctic risks being without a reliable source of fresh water.

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