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River piracy: Caught in the act for the first time...

Rivers love to steal each other's water and carry it on down to its inevitable marine destiny, and the diversion can have major effects on the landscape. The Thames for example used to meet the Seine and drain out through the crowned land of the North Sea, then settled as the sea levels rose to come out at the Wash, just to the north of the Anglia bulge before being captured and draining down its present bed some millennia ago. While geomorphologists (those who study how landscapes come to be) have deduced many such events as having happened, they have finally had an opportunity to study it, courtesy of a warming world.

River capture can happen in many ways, including tectonic uplift or subsidence changing the gradient of slope to the sea and the shape of the landscape, natural dams (eg landslides, lava flows), erosion of the valley head into another valley or into the next lateral one as a river meanders, or in limestone regions where rivers can sink into the ground and reappear in a different valley. The last cause is the culprit in this case: glacial retreat, accelerated by climate change. When this happens one bed is left high and dry while the other is swiftly deepened by the extra erosion, sometimes into a gorge.

Our example happened last spring in the Canadian Yukon, where the meltwater from the Kaskawulsh glacier that fed the Slims River (which was up to 150 metres wide and averaged 3m deep) was suddenly diverted into another streambed over the course of a mere four days between 26-9 May. The warm conditions in the Arctic last year led to exceptionally intense melting, which tipped the drainage gradient in such a way that the second streambed received the flow as a tongue of ice retreated and a new channel out of a glacial lake was opened. The meltwater carved its way through an ice field that had previously blocked it and reached a tributary of the Alsek, whose steeper gradient captured the Slim's water.

The event was monitored by a team of scientists who had been keeping an eye on the glacier for years, since it has retreated by over half a km since the 1950's. They were shocked when they turned up for fieldwork one day to find the river bed that they were measuring mostly empty, and used a helicopter and drones to try and find where the water had gone. At the snout of the glacier they saw that the emerging stream that used to split into two rivers heading in different directions was now directing the waters down just one of them. The second river is a UNESCO World Heritage site popular with white water rafters, and I suspect that future rides will be even more intense than before since it has just doubled in size at source.

While it may seem inconsequential it makes a difference of thousands of kilometres to the location of the water's eventual encounter with the ocean, displacing it from the Bering Sea to the Gulf of Alaska. Fish populations are redistributing and animals changing their patterns to graze the ex river bed, while the chemistry and level of local lakes is also changing in consequence. Two once riverside hamlets are also now high and dry, with likely consequences for their lives and economies. Kluane Lake (Yukon's largest) was fed by the Slims, and sediment cores reveal the moment that the glacier's meltwater began flowing down the Slims some centuries ago as the glacier advanced during the Little Ice Age (see http://bit.ly/1j3khlF).

It also brings to mind the many multilayered changes that await us as our planet inexorably continues to warm. Researchers are predicting more of these events as the temperature rises. There are other far more important rivers passing through major cities to which this could happen. Several of Asia's main rivers (Indus, Ganges, Mekong, Brahmaputra) have their source in the same region, any redistribution of water resources here would have catastrophic consequences (India and Pakistan are already arguing bitterly over their sharing of the Indus). Another at risk area is found in the high Andes, particularly where water starved Peru meets Brazil, but also between Chile and Argentina (whose wine regions are mostly glacier watered oases in the desert). The effects on downstream ecosystems of piracy in large river system would be equally devastating.

Loz

Image credit: 1: : Dan Shugar/University of Washington Tacoma 2: Jim Best/University of Illinois http://bit.ly/2pd1Ted http://bit.ly/2pK8rl7 http://bit.ly/2p1yR5g http://bbc.in/2pxcqV5 Original paper, paywall access: http://go.nature.com/2q3k2zd

#Science #Climate change #Global warming #Kulane Lake #Gulf of Alaska #River #Slims river #the earth story

Kluane Lake Changes

I dropped this blog rather abruptly in April when my ranger job started back up. Living remote like I do, I had limited internet access. But my job ended Friday, and this ranger is rambling again. Follow my journey as I head south.

Traveling down the Al-Can Highway through Canada, one of my favorite parts of the drive is skirting the edge of Kluane National Park in the Yukon. Kluane is our sister…

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#AlCan Highway #climate change #Kluane Lake #Kluane National Park #Slims River #stream capture #winter