First ever observed case of ‘river piracy’ saw the Slims river
disappear as intense glacier melt suddenly diverted its flow into
another watercourse
A view of the ice canyon that now carries meltwater from the Kaskawulsh
glacier, seen here on the right, away from the Slims river and toward
the Kaskawulsh river.
Photograph: Dan Shugar/University of Washington Tacoma
An immense river that flowed from one of Canada’s largest glaciers
vanished over the course of four days last year, scientists have
reported, in an unsettling illustration of how global warming
dramatically changes the world’s geography.
The abrupt and unexpected disappearance of the Slims river, which
spanned up to 150 metres at its widest points, is the first observed
case of “river piracy”, in which the flow of one river is suddenly
diverted into another.
For hundreds of years, the Slims carried meltwater northwards from
the vast Kaskawulsh glacier in Canada’s Yukon territory into the Kluane
river, then into the Yukon river towards the Bering Sea. But in spring
2016, a period of intense melting of the glacier meant the drainage
gradient was tipped in favour of a second river, redirecting the
meltwater to the Gulf of Alaska, thousands of miles from its original
destination.
The continental-scale rearrangement was documented by a team of
scientists who had been monitoring the incremental retreat of the
glacier for years. But on a 2016 fieldwork expedition they were
confronted with a landscape that had been radically transformed.
“We went to the area intending to continue our measurements in the
Slims river, but found the riverbed more or less dry,” said James Best, a
geologist at the University of Illinois. “The delta top that we’d been
sailing over in a small boat was now a dust storm. In terms of landscape
change it was incredibly dramatic.”
Dan Shugar, a geoscientist at the University of Washington Tacoma and
the paper’s lead author, added: “The water was somewhat treacherous to
approach, because you’re walking on these old river sediments that were
really goopy and would suck you in. And day by day we could see the
water level dropping.”
The team flew a helicopter over the glacier and used drones to
investigate what was happening in the other valley, which is less
accessible.
“We found that all of the water that was coming out from the front of
the glacier, rather than it being split between two rivers, it was
going into just one,” said Best.
The Kaskawulsh River, seen here near its headwaters, is
running higher now thanks to the addition of water that used to flow
into the Slims River. Photograph: Jim Best/University of Illinois
While the Slims had been reduced to a mere trickle, the reverse had
happened to the south-flowing Alsek river, a popular whitewater rafting
river that is a Unesco world heritage site. The previous year, the two
rivers had been comparable in size, but the Alsek was now 60 to 70 times
larger than the Slims, flow measurements revealed.
The data also showed how abrupt the change had been, with the Slims’ flow dropping precipitously from the 26 to 29 May 2016.
Geologists have previously found evidence of river piracy having
taken place in the distant past. “But nobody to our knowledge has
documented it happening in our lifetimes,” said 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.”
Prof Lonnie Thompson, a paleoclimatologist at Ohio State University
who was not involved in the work, said the observations highlight how
incremental temperature increases can produce sudden and drastic
environmental impacts. “There are definitely thresholds which, once
passed in nature, everything abruptly changes,” he said.
Between 1956 and 2007, the Kaskawulsh glacier retreated by 600-700m.
In 2016, there was a sudden acceleration of the retreat, and the pulse
of meltwater led to a new channel being carved through a large ice
field. The new channel was able to deliver water to the Alsek’s
tributary whose steeper gradient resulted in the Slims headwater being
suddenly rerouted along a new southwards trajectory.
In a geological instant, the local landscape was redrawn.
Where the Slims once flowed, Dall sheep from Kluane National Park are
now making their way down to eat the fresh vegetation, venturing into
territory where they can legally be hunted. The formerly clear air is
now often turned into a dusty haze as powerful winds whip up the exposed
riverbed sediment. Fish populations are being redistributed and lake
chemistry is being altered. Waterfront land, which includes the small
communities of Burwash Landing and Destruction Bay, is now further from
shore.
Sections of the newly exposed bed of Kluane Lake contain
small pinnacles. Wind has eroded sediments with a harder layer on top
that forms a protective cap as the wind erodes softer and sandier
sediment below. These pinnacles, just a few centimeters high, are
small-scale versions of what are sometimes termed “hoodoos.” Photograph:
Jim Best/University of Illinois
A statistical analysis, published in the journal Nature Geoscience,
suggests that the dramatic changes can almost certainly be attributed
to anthropogenic climate change. The calculations put chance of the
piracy having occured due to natural variability at 0.5%. “So it’s 99.5%
that it occurred due to warming over the industrial era,” said Best.
The Yukon region is extremely sparsely inhabited, but future river
piracy could have catastrophic effects on towns, villages and ecosystems
that have sprung up around available water, according to an analysis
accompanying the paper, by Rachel Headley, a geologist at the University
of Wisconsin-Parkside. “If a river changes course so drastically that
the drainage basin no longer reaches its original outlet, this change
might eventually impact human and biological communities that have grown
around the river’s original outlet,” she said.
Thompson, who has documented glacial retreat on Mount Kilimanjaro,
predicts that there will be an acceleration in the observations of river
piracy events as glaciers retreat globally.
“I think we could see similar divergence in streams in the Himalayas as well as throughout the Third Pole region,
the Andes of Peru, other sites in northern Canada and Alaska,” he said.
“Often these events occur in remote and poor parts of our planet and
thus go largely unnoticed by the larger population but greatly impact
the livelihood of many families downstream.”
