Long exposures give photographers the ability to create an alternate reality. This is perhaps most prominent in waterfall photography. That beautiful appearance of silky water is typically captured with a slow shutter speed. This can be anywhere between one second, and 1/60th of a second. Couple this with a graduated ND filter, and you can create something dramatic with dark clouds and a well lit foreground.
For this type of work, a tripod is super helpful. Also, overcast days are ideal for waterfall photography. The clouds work as a natural softbox, providing diffused even light. This is far more pleasing than a scene that's dappled with sun and shade. With a little bit of planning you can increase your chances of success. I try to arrange my waterfall shoots on a partly cloudy day, or very early in the morning when the sun is still low in the sky.