Hope this little paintover gets my point across. But basically one the key things of doing any sort of environment (and almost drawing anything) is getting a good sense of depth, and thus creating a space. You can do stuff like perspective and atmospheric perspective, but with natural environments its pretty much a mix of perspective and overlapping that create depth. If you have clear areas where objects overlap - and then they are scaled correctly - it immediately creates the idea of the depth. The idea of overlapping - especially in your picture is the verticals across the basic horizontal of the river. Try to think of the river as a completely flat plain so all lines/strokes flow horizontally and then everything is jutting out of it so to speak.
You picked a pretty difficult picture in as much the left hand side of the picture is not very well defined, many of the rocks are of similar colour/tone and the tree in the foreground also merges with it. But your main objective should be simplifying out all the big basic shapes and establishing them in a space.
I know this is all pretty obvious but sometimes its easy to skip over the easy stuff. Sometimes its better to try and keep it simple.
Hope this helps you out... and makes sense.
This is over the original photo too btw.
Edit - damn, I broke a new page. Sorry.
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