Why are the banks of the Yukon River white?
Exploring the neighbourhood I am renting in, I find a well-worn trail that winds through a patch of boreal forest. It leads me toward the Yukon River, to a steep bank overlooking the river. The vegetation is shrubby, dry species, lots of fireweed, and the trail has become … sandy? The cliffs I am standing on as I look out at the mountain ranges surrounding Whitehorse are made of sand. But why? I look across the river to see more cliffs, white, sandy cliffs. I don’t know much about the geology of this region, but I am surprised by the accumulation of sand here. So of course it was the first thing I looked up upon returning home!
The Yukon River begins in the glaciated coastal mountains of British Columbia, running up through the Yukon and into Alaska. The name Yukon River is likely derived from the Gwich’in word Yu-kun-ah meaning “great river” or “big river”.
Going wayyyy back - like 250 million years back - there was a tropical ocean in the region, which formed the limestone Canyon Mountain. Over time this limestone has been weathered down, with the formation of crevices and caves across the range; and lots of eroded silts and sands.
About 20,000 years ago an ice sheet covered the south of Yukon territory. As the ice sheet began receding about 15,000 years ago it left behind weathered mountains, kettle lakes, and glacial streams. These streams carried large amounts of the eroded silt and sand downriver, accumulating up to 90 metres in height.
It is these silt and sand deposits, which have been exposed over time, that make up the white, sandy bluffs I saw. The banks of the Yukon around Whitehorse are white because of the glacial limestone silts left behind thousands of years ago!
Along parts of the river can be seen a thick white band of volcanic ash that was deposited some 1200 years ago from an eruption of Mount Churchill. This layer is generally between a few centimeters and half a meter in thickness.
You can learn more at this link.
https://www.cgenarchive.org/whitehorse.html
The Yukon River, image taken by Samantha Mitchell
Bluffs along the Yukon River near Whitehorse, image taken by Samantha Mitchell