One of the almost overdone photographic effects is flowing water, which is still a little tricky to get right. You need to be able to get a shutter speed in the 3-to-4 second range, which is becoming difficult with modern high-ISO digital cameras. The secret here is a Neutral Density (basically a dark gray) filter over the lens, or to shoot early or late enough in the day that the natural light is subdued enough to let your shutter speed go long.
I used a combination of techniques to get this shot: I shot at 7 AM, before the sun was fully up; I used a polarizer to further reduce the amount of light entering the lens; and I shot at a high f/stop to slow the shutter speed even more.