Thats pretty impressive..

Yeah, most of the water in the river/falls looks to be real footage shot on the river with actual barrels going down the river.. I did see quite a bit of layering, where they selected a layer above the river footage and green screened in CGI characters, who have to be positioned and rotated perfectly to account for the camera angles and whatnot.. so, like you said, a combination of real river footage and CGI layered on top.. I only watched it once and very briefly, but thats my initial take.. if I get a chance to look more closely, I may be able to "render" (pun intended) more thoughts on the subject..

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Friendship is like peeing your pants.. Everybody can see it, but only you can feel the warmth..