What is river piracy?
For those who might not know what the term “river piracy” means, it refers to the reorientation of streamflow from one channel to another, resulting in mixing two different rivers.
First study case documented in the human timescale:
The Kaskawulsh glacier’s located at the border between Alaska and Canada has recently led river piracy to occur in only a few days in Spring 2016.
The Yukon area is a perfect example of rapid proglacial drainage changes mainly been studied over Quaternary or longer timescales.
The Yukon area is a perfect example of rapid proglacial drainage changes, and its consequences on ecosystems, lakes and morphology of rivers.
Melted water from the Glacier used to be drained into 2 main paths (the yellow lines on the satellite picture reflect both of these pathways):
Northward to the Bering sea via the Yukon river: first water was flowing from the toe of Kaskawulsh glacier via Slims river, distributary of one of the biggest lake of Canada, Kluane lake, main source for Yukon river.
Southward to the Pacific Ocean via the Alsek river: the rest of the melted water was taking Kaskawulsh river path, main distributary for Alsek river.
The melted water issued from the glacier flowed via Slims River into Kluane Lake, the largest lake in Yukon and part of the Yukon River watershed. Thinning and retreat of the glacier, caused by the current Climate Change triggered the river piracy. The satellite images show the rapid change in water flow between 2015 and 2016.
Glacier retreat due to Climate Change
Evidence for the glacier retreat due to Climate Change has been found through a study of recent volume and area changes of Kaskawulsh Glacier, based on satellite images analysis.
This image is an undeniable evidence of the glacier retreat since the 70's as it's size decreased over time (see the coloured lines).
Terminal lakes appeared from 1972 due to the glacier retreat. Those have grown since that year, which is another evidence for Climate Change- Driven glacier retreat. This image actually shows how two-terminal lakes (Slims and Kasakwulsh) have been expended due to glacier melting since the '70s.
Formation of dead ice at the toe of Kaskawulsh glacier led flow to change of direction and acted as a wall to Slims river. The dead ice is actually a part of the glacier that has collapsed. As it doesn't flow anymore, glacial and fluvial sediments cover the dead ice with time and can act as a barrier for stream flows.
Geomorphological consequences of river piracy
The reduction of Slims River flow had and will have substantial effects at Kluane Lake:
The water level has fallen and will fall further in the future, leading to a closed basin. This youtube video clearly shows that the water level dropped, uncovering fluvial sediments.
Also, the sediment supply from Slims River has stopped with effects on the structure, chemistry and lake ecosystem: dust storms are now occurring daily due to the low water level. Cracks also appeared on the ground with contact to the dry fresh air of high latitude landscapes.
Big geomorphological changes may occur in the future due to change in flow energy:
Slims river may change to a meandering river system due to the decrease on sediment transportation. This will lead to the formation of several oxbow lakes on the future geological timescale.
Kaskawulsh river may see an increase in sediment transport and bank erosion. Modelling studies also predict this river will end up flowing in the Kluane lake.
Conclusion
River piracy is still considered as an unusual consequence of glacier retreat. Still, many studies are focused on sea-level rise or ancient glacial environments. We may expect river piracy to become more common in the future due to rising precipitation and temperature affecting the world's biggest glaciers. Such phenomenons have big impacts on the surrounding environments (geology, ecosystems, human activities).
This area should be monitored closely in the near future as big changes in ecosystems and biodiversity are expected to happen in this region.
Watch the space, further posts to come!
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