Well, of course, the green and blue and turquoise are reflecting the sky. Brown rivers have drainage from soil. Such as farming upstream. Clear water will flow eventually anywhere, but particularly of mountain and rocky rivers, usually small ones. Big rivers with boat traffic may look blue because they reflect the sky, but are usually brown or transparent brown. Green may indicate algae, or plants, but usually the blue-green is a combination of reflected or refracted light and algae.