What is the first thing you focus on in this picture?
First of all, wow. What a pretty landscape. It is just too bad it isn't a double rainbow, then this picture would have been something truly special.
Okay now that you are all done appreciating this beautiful view, lets get down to business. This picture is a very good example of the monocular cues of depth perception.
Relative Size: we can tell relative size from this picture because obviously the rocks in the foreground of the picture aren't the same size as the mountains in the background, but they appear to be that way.
Texture Gradient: Look at the grass. In the foreground you can easily make out all of the blades but as it goes further back, it just becomes a sea of green. spooky.
Interposition: We can tell that the snowy mountains are at the very back of the frame here because we see the other two mountains cutting off some of the snowy mountains. However, we don't know exactly where the rainbow is located because there is nothing blocking some of our view of it.
Linear Perspective: the photographer definitely knew about this principle because if you follow the lines from the two mountains on the sides, they will converge somewhere in the center of the picture. That gives the picture a good, whole, complete feeling.
Height in Plane: Close things are lower, distant things are higher. I think this is pretty self evident.
Light and Shadow: Personally, I really like the shadows in this picture. Essentially, there are three different mountains here, a shadowy one, a snowy one, and a sunny one. Also the shadows on the rocks in the front really give it a 3D feel.