Look beyond the obvious waterfall picture and think about how water reflects colour, especially over long exposures. White water will reflect blue in after-dusk shots but for something different, look at the less turbulent parts of a river.
Nikon D2x, 200mm, 4sec @ f/16, ISO 100 Tim Gartside
In autumn, many leaves may be present in the water column and during a very long exposure they may alter the appearance of the river altogether. Use a small aperture or neutral density filter (or both) to gain a longer shutter speed without overexposing. A polarising filter will also give you about one stop.
Nikon D2x, 12-24mm, 8sec @ f/13, ISO 125 Niall Benvie
For an alternative view, you can illuminate the water using flash to freeze its motion, while allowing daylight to light the surroundings, which will be blurred if you handhold the camera during a long exposure.