Why is clear mountain stream water green?

Question

Why is water that is otherwise pristine in a mountain creek, lake, or river have a green tint to it in deep pools? I notice this a lot living in the Appalachian mountains and have also seen it when I lived in Colorado.

Answer 1

The only thing that I can think of that would cause the water to look green would be that their is algae in the water or that you are seeing a reflection of the green in the surrounding countryside.

Answer 2

Algae (a form of plant life) growing in streams can actually cause all of the water to have a greenish tint, however it is likely most likely noticeable in large pools (the more water you look through, the more green particles there are to color it).  In addition, still water usually allows more algae to grow, so quiet pools can accumulate more than areas where the water is flowing rapidly.