You may ask. Actually, I have shot enough waterfalls to know what shutter speeds work best with what lens I'm using. The real trick is to get slow enough speeds without blowing out the highlights. Sometimes cranking the f/stop way up, sometimes using a circular polarizer.
Oh, because I read that when shooting waterfalls such as the one you shot, getting a meter reading from the green around it will make it the best. You get your reading for the foliage, and set it at a -2/3 exposure, then recompose.
I understood it. I haven't practiced it but I understood. I generally try to set my shutter speed to 1/3 second on my wide lens, and 1/5 second on my VR. It generally gives me the effect I'm looking for. Of course, there are always variables.. I also try to shoot at dusk if possible.. Sometimes I will manually focus because auto will sometimes focus on the surface of the water instead of the rocks under.