Before I tell you how you can do this, a little science. Stop for a second and think of what's really happening at the waterfall. Water is comprised of individual molecules which attract each other. Together, they try to form a shape with the least surface area possible. That's the flow you see in the river or stream as it approaches the falls.

Once the falls are reached, the shallow depth and irregular surface of the stream bed breaks the water's surface tension and individual drops begin to form. The flow over a waterfall is chaotic. Science isn't smart enough today to see the waterfall and predict where any individual drop will be as it begins its descent. Drops are crashing into each other or bouncing off rocks. No two take the same exact path.

If you take a photograph with a fast shutter speed that's what you'll see, the chaos of those individual drops. This is the classic photo (the second one above) that most cameras set to AUTO on a sunny day will produce. My heart cries for you when you set your camera to AUTO, but that's another story.

To get the dreamy photo (the top picture) you've got to slow the shutter speed. My shutter was open a full two seconds! Now, instead of seeing individual drops, you're looking at the blur the drops motion produces. It's awful if people or still objects are blurred in your photos, but for moving water it's wonderful. You're eliminating the chaos within the flow and instead seeing the average.

The problem is as you open the shutter for a longer period more light gets into the camera's sensor. On most cameras you can't set the shutter slow enough without overwhelming its electronics. Your shot will be overexposed. I overcame that problem with a neutral density filter. This filter cuts the intensity of light without changing anything else. Think of it as welder's goggles for your camera. Mine came from EBay, shipped from India and costing less than $10. I'm sure purists will tell me I've given up some quality by buying a cheap filter. I'll take my chances.

Your photo is based on blur, but you want it under your control. Only things that are moving should be blurry. Your camera shouldn't change that. Any time you shoot with a slow shutter you need a tripod, or just set your camera on an inanimate object--even the ground.

There are no hard and fast answers in photography. This method works with any flowing liquid. Experiment. You'll be surprised by what you can capture.