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River Piracy drains Yukon river almost overnight. Here's how

Caroline Floyd

Monday, April 17, 2017, 3:36 PM - You know climate change is getting serious when rivers are resorting to piracy.

Canadian geomorphologist Dr. Daniel Shugar and his team headed to the Yukon last year to study changes in the flow of the Slims River, only to find out the river was gone.

The Slims, which was fed by the Kaskawulsh glacier, has become the victim of the first case of what's known as river piracy in modern recorded history.

Stock image of the Slims River Valley, Kluane National Park.

In the past, river piracy - the technical term for the waters of one stream somehow diverting to flow into another stream bed - has generally been studied on Quaternary time scales as something that shows up in the geologic record, happening slowly as rivers carve their surroundings over thousands of years.

Until now.

In studying the disappearance of their river, Shugar and his colleagues found river level gauges at the mouth of the Slims where it fed into Kluane Lake dropped abruptly over just four days at the end of last May.

"Where there had been blue, shimmering shallow water, it was now exposed sediment," James Best, one of the paper's co-authors, told CBC News. "The afternoon wind in that area was picking up that sediment and dust was creating a huge dust storm."

The team's investigation soon turned up the culprit - the retreat of the Kaskawulsh Glacier, which has been retreating thanks to more than a century of climate warming.

Prior to May of last year, the glacier had been supplying water to two watersheds and feeding multiple rivers; the Kaskawulsh River, which drains to the Pacific Ocean via the Alsek River, and the Slims, which flowed north to the Bering Sea via Kluane Lake.

During the last days of May 2016, melt water at the base of the glacier finally managed to eat through the thinning ice sheet, opening a new canyon and sending the Slims' share of the water into the Kaskawulsh instead.

Thanks to this abrupt change, water from the glacier that used to flow north to the Bering Sea has changed direction and flows toward the Pacific, instead, leaving the Slims basin high and (mostly) dry.

"[The Slims] was beheaded, if you will," Shugar told Popular Science.

And, as beheadings usually are, it seems this one is likely permanent. In their paper, released Monday, Shugar et al. make the following conclusion:

It thus seems likely that Kaskawulsh Glacier experienced unusually high surface melt in the spring of 2016, which lead to development of an ice-walled canyon and increased flow in Kaskawulsh and Alsek rivers. The ice-wall led channel that was established across dead ice at the terminus of Kaskawulsh Glacier was rapidly enlarged by melt water and localized collapse of the channel walls in the summer of 2016, resulting in the 17 m lowering of Slims Lake. Even if the current canyon walls were to collapse and temporarily block the flow of melt water into Kaskawulsh Lake, the blockage could not pond water in Slims Lake to the level required to re-establish Slims River.

Hydrographs for Kluane Lake, Alsek, and Slims Rivers. Courtesy Nature Geoscience. Graphs show how, prior to 2016, the flow of the Slims and Alsek rivers were similar, whereas in 2016 the Alsek flow dwarfs the measurement of the Slims.

The researchers directly link the change in the rivers to human-caused climate change and its impact on glaciers, which Shugar told Popular Science, "is happening much faster than can be explained by natural causes."

"These effects highlight the consequences of climate-induced glacier retreat," the authors say in the paper's conclusion. "We contend that radical reorganizations of drainage can occur in a geologic instant, although they may also be driven by longer-term climate change.

"Most studies of the effects of climate change on glacial environments deal with enhanced melt of contributions to sea-level rise. We suggest that the effects can be more far reaching."

Sources: Nature | Science News | Globe and Mail | CBC News | Guardian

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