The Nymph is concerned with growing old in its reply to the shepherd. The rivers rush in the spring because the snow from winter is melting. Spring implies rebirth. However, rivers rage because of storms and storm run-off during the fall and winter. Fall and winter imply growing older and dying. As the nymph points out:
" But could youth last and love still breed,
Had joys no date, nor age no need
then these delights my mind might move
To live with thee and be thy love."
In other words, if she could say young, she might agree to the shepherd's request. but the storms of life will eventually age her and she will no longer be as interested in or interesting to the shepherd.