SAN FRANCISCO, April 19 (Xinhua) -- The massive Kaskawulsh Glacier in northern Canada has retreated about more than a kilometer up its valley over the past century, and its retreat triggered a geologic event at relatively breakneck speed last spring, causing a river to disappear.
The toe of ice that was sending meltwater toward the Slims River and then north to the Bering Sea retreated so far that the water changed course, joining the Kaskawulsh River and flowing south toward the Gulf of Alaska, according to a study led by the University of Washington Tacoma and published in Nature Geoscience.
It was the first known case of "river piracy," also known as stream capture, in modern times, which can happen due to tectonic motion of Earth's crust, landslides, erosion or, in this case, a change in a glacial dam, as one of the less-anticipated shifts that can occur in a changing climate. "Geologists have seen river piracy, but nobody to our knowledge has documented it happening in our lifetimes," said lead author Dan Shugar, a geoscientist at the University of Washington Tacoma. "People had looked at the geological record -- thousands or millions of years ago -- not the 21st century, where it's happening under our noses."
The Kaskawulsh Glacier is retreating up the valley because of both readjustment after a cold period centuries ago, known as the Little Ice Age, and warming due to greenhouse gases. A study published in 2016 by UW co-author Gerard Roe shows a 99.5 percent probability that the glacier's retreat is showing the effects of modern climate change.
Shugar and co-authors Jim Best at the University of Illinois and John Clague at Canada's Simon Fraser University had planned fieldwork last summer on the Slims River, a geologically active system that feeds Kluane Lake in the Yukon. When they arrived in August, the river was not flowing. River gauges show an abrupt drop over four days from May 26 to 29, 2016. By late summer, "there was barely any flow whatsoever. It was essentially a long, skinny lake," Shugar said. "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 researchers got permission to use their mapping drone to create a detailed elevation model of the glacier tongue and headwater region, and the result is a geological postmortem of the river's disappearance.
"For the last 300 years, Slims River flowed out to the Bering Sea, and the smaller Kaskawulsh River flowed to the Gulf of Alaska. What we found was the glacial lake that fed Slims River had actually changed its outlet," Shugar said. "A 30-meter (100-foot) canyon had been carved through the terminus of the glacier. Meltwater was flowing through that canyon from one lake into another glacial lake, almost like when you see champagne poured into glasses that are stacked in a pyramid."
That second lake drains via the Kaskawulsh River in a different direction than the first.
The geologic event has redrawn the local landscape. Slims River crosses the Alaska Highway, and its banks were a popular hiking route. Now that the riverbed is exposed, Dall sheep from Kluane National Park are making their way down to eat the fresh vegetation, venturing into territory where they can legally be hunted. With less water flowing in, Kluane Lake did not refill last spring, and by summer 2016 was about 3 feet, or 1 meter, lower than ever recorded for that time of year. Waterfront land, which includes the small communities of Burwash Landing and Destruction Bay, is now farther from shore.
On the other hand, the Alsek River, a popular whitewater rafting river that is a world heritage site designated by the United Nations Educational, Scientific and Cultural Organization (UNESCO), was running higher last summer due to the addition of the Slims River's water.
Shifts in sediment transport, lake chemistry, fish populations, wildlife behavior and other factors will continue to occur as the ecosystem adjusts to the new reality, Shugar said, noting that "the event is a bit idiosyncratic, given the peculiar geographic situation in which it happened, but in a broader sense it highlights the huge changes that glaciers are undergoing around the world due to climate change."
"So far, a lot of the scientific work surrounding glaciers and climate change has been focused on sea-level rise," he was quoted as saying in a news release from UW. "Our study shows there may be other underappreciated, unanticipated effects of glacial retreat."
