A lake I fish when at normal pool(or close to) had a Water Willow type emergent vegitation that grew along the bank line out to a max depth of 3-3.5fow up in the river! My question is how long does it typically take after a lake comes back up from 8ft low to normal pool to grow back... If it will at all?!

It will start coming back next year but will be in just a few spots up and down the river. You may find the fishing to be even better in those few spots because of the isolation. It is all the shade they have to go to. On the other hand every fisherman that goes up the river will hit those few isolated spots. That part is not good. When the bank gets covered with them again it become a technique to fish the Water Willow. Not many people will even fool with a long stretch of it unless they are hitting the mother load on the head.

It will gradually spread and the banks will be covered again in two years. It is there just dormant without the sun light. It does similar to what other vegetation does which is put out small leafy stringers to the surface. This is what you will see first nest year and it will not look anything like the Water Willow that you are use to seeing. Those stringers will give it the light it needs to get established again and by the end of the Summer there will be cluster growing again.