In The Revenant (2015), there's a scene where Leo finds fish in the river and the fish is trapped in rocks which are arranged in a U shape on the side of the river. So did someone arrange the rocks in a U shape or were they natural arranged?
He made that rock formation himself to catch fish.
The arrangement of rocks into that shape is a traditional fishing technique called a fishing weir. Quite a few Native American tribes used this kind of fishing technique to catch freshwater fish. The rock formation forces the fish swimming downstream into a small area where they can't easily escape, and can be grabbed by the person fishing.
From what I've read, it's actually not likely that a real fisherman would have used a weir in such a shallow stream; in those cases you could typically just grab the fish by hand as they swam in front of you. Nonetheless, it is a valid fishing technique in other cases.