When considering the nymphs living in a stretch of river it is important to know that not all nymphs are the same. Just concerning mayfly's they can be subdivided into the following
Clingers - position themselfs on rocks or organic matter and feed on organic matter or passing food from that position. they may move to a new location once the food they are clinging to is gone
Burrowers - dig a hole in the substate of the stream bottom and feed in that position
swimmers - swim around and feed in the current. these nymphs do not only move when molting but all the time.
I just attended a presentation by author Ann Miller and she had an amazing amount of info about the life cycle of mayfly's and other aquatic bugs that most fly fisherman don't know. It really enlightened me to the advantages of knowing what bugs live in the particular stretch of river your fishing. this could be a totally different food source for the local fish by moving a hundred yards up and down the stream.
-Jeff