I’ve often been asked “how do you choose your shutter speed when photographing waterfalls?” The answer to that is I cheat! by that, I mean I have a ball park figure – typically 1/3s but I take a series of shots at varying shutter speeds from maybe 1/20s to 3s. At the shorter shutter speeds the look and feel of the image varies greatly as the shutter speed is changed, however, once over 3s the differences become harder to spot as the water blurs into a silky whiteness. The longest exposure I took at this location was 3s but that had some camera shake visible, I probably caused that, but the fact that the tripod was in the water and flowing over it’s legs doesn’t help.
Personally I prefer the motion of the water in the 3rd image, 0.4s – which is close to my typical choice of 0.3s. I not a great fan of the 10s plus silky water pictures that are very common, it works on some images but is often over used, but many people love them which is the important part.
To achieve the long open shutter time required it is preferable to use Neutral Density Filters of about 4 stops or so, a 1.2 being 4 stop. 0.3 = s stop, 0.6 = 2 stop, 0.9 = 3 stop etc.

I have included 5 images decreasing in shutter speeds from 1/10 of a second to 1.6s.

0.1sone tenth of one second 1/10

1-6thone sixth of one second 1/6

0.4sfour tenths of one second 4/10

1sone second 1.0s

1.6sone point six seconds 1.6s