When Jim Best and Dan Shugar visited the Slims River in Canada’s Yukon last summer, they made a discovery that caused the world to sit up and pay attention to this remote corner of North America. Instead of a flowing current, the pair found only a dry and dusty river bed. ‘The river had virtually dried up, and the lake into which the river flowed had dropped in water level,’ recalls Best, Professor of Sedimentary Geology at the University of Illinois.

They later discovered that the water had rerouted into a different river system – a phenomenon with the somewhat dramatic name of ‘river piracy’. Instead of flowing along the Slims river, through Kluane Lake, and eventually out into the Bering Sea, the water now travels primarily along the Kaskawulsh river, then south into the Alsek river, and ultimately into the Pacific Ocean.

Such a transformation has occurred numerous times throughout the planet’s geological history – often due to gradual erosion or the movement of a fault – but has never been observed to occur as suddenly, happening over just a few days in May 2016. ‘Geologists have seen [evidence of] river piracy before, but nobody to our knowledge has documented it actually happening [within] our lifetimes,’ explains Shugar, Assistant Professor of Geoscience at the University of Washington, Tacoma.

The dramatic switch was caused by the rapid retreat of the Kaskawulsh glacier – thanks to climate change – which caused the flow of the meltwater to be redirected, and prompts questions about the impact it could have on the surrounding Yukon territory. Best points out that while much of the southern part of the territory is ‘sparsely populated’, and therefore potential flooding caused by the extra water is unlikely to cause any ‘real human impacts’, the opposite issue could be a cause for concern further north.

‘If Kluane Lake levels go down,’ he predicts, ‘the lake could thus have no inflow and no exit flow, which would radically alter lake water nutrients and circulation, and this may impact on the lacustrine ecology. In addition, if the lake outlet were to dry up as a consequence, this river would be dry or far lower and thus the few habitations along it would be affected.’

This was published in the June 2017 edition of Geographical magazine.