River Piracy sounds like an exotic form of stream based pillage and plunder, but rather refers to the reorientation of stream flow from one channel to another. Also known as stream capture, the causes vary, and include tectonics shifts which changes slope, natural dams (landslide or ice), headward and lateral erosion, karst topography, and glacial retreat. Notable ice dams have diverted rivers, of note is the River Thames, which was shifted 450,000 years ago cause it to
A recent set of stories about the Slims River in the Yukon territory illustrates the last of these points, where the retreat of the Kaskawulsh glacier and it’s shift from the Slims River to the northwest and into the adjacent Kaskawulsh River to the southeast. The phenomenon isn’t uncommon, but the pace in which this ‘theft’ occurred is notable: “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 image below shows the “aerial view of the ice canyon that now carries meltwater from the Kaskawulsh Glacier, seen here on the right, away from the Slims River and toward the Kaskawulsh River.”
A map of the area shows the relationship of the river and the shift, which short-circuited a longer path through the Kluane River into the Yukon River and eventually the Bering Sea, now connecting to the Alksek River and flowing into the Pacific Ocean. From a global perspective the flow doesn’t mean much, but on a local scale the impact is more acute.
From the article in the Globe and Mail, “Climate change stole a Yukon river almost overnight, scientists say. Here’s how“, which mentions the process at work:
“That water now flows into the Kaskawulsh River, a tributary of the Alsek, which runs southward to the Pacific. Following this route, it reaches the ocean some 1,330 kilometres away from where it would otherwise have ended up. Signs of the rerouting have been observed on both sides of the mountainous divide. Gauges on the Alsek River reveal that it experienced a record discharge last year. Because the river mostly flows through parks and protected lands, the increase has had no immediate human impact. On the Slims side, the effect of water loss is more obvious. Last summer, Kluane Lake dropped a full metre below its lowest recorded level for that time of year. The reduced inflow from the Slims spells a huge change for the 65-kilometre-long lake, with implications for nearby communities and visitors who access its waters for fishing and other activities.”
The quality of the Lake ecosystem is one issue also, as mentioned in the Guardian article “The river stolen by climate change”, quoting scientist Jim Best: “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.’”
The dry valley left over, in this case the Slims River, is referred to as a wind gap, and scientists have discussed the potential issues erosion and dust storms. As noted in the CBC story, “Retreating Yukon glacier makes river disappear“, the river is: “…prone to dust storms. “It’s certainly not unusual to see rapid drainage changes in and around these glaciers. It’s a common situation,” Bond said. “Until vegetation really starts to stabilize that floodplain, it’s going to be a dusty place, I’d imagine … It will be a really interesting study to see how that floodplain evolves in the next ten years or so.”