If you are talking about taking a still photograph of a waterfall that would "blur" the flow of water, use a neutral density filter (4X or 8x) and a tripod. Start with a slow shutter speed like 1/4 second, check your results, and adjust it until you get the desired effect. With the neutral density filter, you should be able to select a lens opening and ISO combination that will give you the shutter speed you need.
|High Altitude Rocky Mountain Railroad by cjf2|
from On the Rails...
|Evening at the lake. by Murat ÜNSAL|