Waterfalls don't appear to be flowing down behind glass

I expected the water in a waterfall to look like it was flowing down when behind glass blocks.

What actually happened was the water behind the glass looked like it wasn't moving

Steps to Reproduce:
1. Place glass blocks directly in front of a waterfall (or any downward flowing water)
2. Look at the water through the glass block

It's a slight difference, but noticeable if you look closely. The water appears to be moving a little but that's due to the flowing on the other side of the waterfall.