This rare weather phenomenon would be due to a combination of cold, dense air and an eddy in the river.
For their latest movie 'Frozen', Disney has created a snow simulator to improve realism and ease the work, it's simply stunning.
The future of physics in gaming and animation looks promising!
Skip to 0:55 if you're not interested in the explanations.
Scotty Bob flew through a very narrow canyon (the Beehive line) in Utah and this is what he filmed via his helmet cam.