What the hell? People are really believing that? Sorry to be skeptical but just look at the "mammoth" movement in the water. He is clearly passing through parts of the river where the stream is variable but its balance is totally constant and so is his speed.

Also I am always amazed that people filming something like that provide 5 seconds clip. "Oh cool a mammoth I am going to shoot for 10 seconds and stop randomly, because you know, Siberia is busy."