Does anybody know where this movie scene was filmed?
It's from the first 5 mins. It looks amazing how it is a forest with a creek running through.
If it was filmed at the same location as the rest of the opening battle, then you are looking for the Indian Morley reserve, part of the Stoney Nation. Cannot comment on the exact point where the creek runs unseen under the trees however, so sorry for half an answer.
Following the river near the the Indian Morley reserve on Google maps I found this location which looks like the location of the opening battle to me. Google map link - https://email@example.com,-114.909539,441a,35y,90h/data=!3m1!1e3