my take...
Low water means clear water(usually) which means tougher fishing(usually) which also will limit the amount of fish coming up the river if you're talking about salmon or steelhead I guess. Once the water comes up a little and gets a little color, you'll get a little better fishing(less visibility/less line-shy fish) plus the increased flow will usually trigger fresh fish coming from the salt.
I will also say that if you ask 5 different fishermen you'll get 5 different opinions.

This would all apply till the rivers are totally blown at which point all you can do is watch lumber floating down the river.