I have only watched the movies and I am currently reading The Fellowship of the Ring (haven't finished it yet, I am at the Bridge in Khazad-Dum).
A number of times we see characters such as Gandalf, Aragorn or Glorfindel describe how lethal a foe the Nine are. I also read that the Nine ride horses taken in tribute from lands such as Rohan and they only bother with the black cloaks to give some form to their nothingness when they go out in the world of the living.
But the moment Frodo crossed the Ford at Bruinen (and the subsequent flooding of the river), we see the same characters talking about how the Nine will not bother them for some time because apparently they have lost their horses. It seems bizarre to me that such terrible an enemy be dismayed by mere loss of mounts. I understand that the Nine cannot see in the light (but their horses can) yet that doesn't seem to really hamper them as they utilise other senses to track their target.
So why do the people assembled at Rivendell seem to think that just because the Nine have lost their mounts, the threat is somehow temporarily over? They know the flood couldn't have killed or hurt the Nazgul. They know that they have no physical form to begin with that might have been hurt in the flooding. So what gives?