Canon XSi, 100mm macro lens, 30 seconds, f/32
As you might imagine, the stream did not really look like that. It was simply water flowing down and around the rocks. So, how did I get the water to look like that? Did I use some Photoshop magic? No, I didn't (as a matter of fact, I don't even own Photoshop).
The trick to taking "ghostly water" shots like that is to use a long exposure. If you look under the picture, you'll see that for this shot, I left the shutter open for 30 seconds. That's quite a bit of time. Because the water was flowing at a nice pace (had it been flowing faster, the water would have looked even more "ghostly") leaving the shutter open for so long allowed me to capture much of the movement, resulting in the image you see.
Of course, it's important to remember that if you're going to leave the shutter open for that long, there are two things you MUST do:
1. Use a tripod. I don't care if you're the best surgeon in the world -- no one can hold their hands still for 10 seconds, let along 30. You absolutely must use a tripod to keep your camera still while the shutter is open.
2. Change the f/stop on your camera. I stopped the camera all the way down to f/32 -- the smallest aperture I could get with the lens I used. If you don't do this, your entire picture will be completely overexposed.
3. Although not a must, a filter would also help to reduce the amount of light coming into your camera. This will allow you to keep the shutter open longer.
As always, I welcome all comments, critiques and criticisms.