Receding
glacier causes immense Canadian river to vanish in four days
First
ever observed case of ‘river piracy’ saw the Slims river
disappear as intense glacier melt suddenly diverted its flow into
another watercourse
Hannah Devlin
Science correspondent
@hannahdev
Monday 17 April 2017
16.00 BST Last modified on Monday 17 April 2017 23.47 BST
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.”
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.”
Sem comentários:
Enviar um comentário