Is this scene happening on Earth? I mean, perhaps this scene was made completely with CG effects. But if not, which waterfall looks like that? Please, answer with images and sources.
It was filmed at Dettifoss.
Scott used Dettifoss, in Iceland, as the filming location for the scene (many thanks to @Paul D. Waite for the link). The links provided show that very little except the spaceship was changed in post-production.
But its location in the fiction of the story is deliberately undetermined.
From this movies.com interview with Ridley Scott:
Movies.com: That is our planet, right?
RS: No, it doesn’t have to be. That could be anywhere. That could be a planet anywhere. All he’s doing is acting as a gardener in space.
So it could be Earth, but it might not be, and it doesn't matter to Scott or the story; that scene exists as a generic representative of a process which occurred on an unknown-but-large number of worlds (including Earth) in much the same way.
And it's certainly not meant to be Dettifoss, Niagara, or any of the world's other current waterfalls. 3.5 to 4 billion years ago --roughly when life first showed up on Earth-- Niagara Falls didn't exist (it was first dug out by glaciation 10,000 years ago), and neither did the crust on which all of our modern waterfalls sit.