plans to lavish on the woman whom he wants to  "Come live with [him] and be [his] love".  The whole poem turns the silly promises that the Shepherd tries to make (pleasures of the landscape, sitting by shallow rivers and waterfalls, birds singing, beds of roses, etc.) to the nymph into realistic retorts by a practical mistress.  Thus, Marlowe's bucolic scenes of comfort and beauty in nature are turned on their heads by the nymph's replies -- "When rivers rage and rocks grow cold" (line 6) is an example.

The word "rage" in this instance means the movement of the river, swollen and fast-running, foaming and rushing over rocks between its banks.  This is not a word being used to describe human anger in this instance (although this may prefigure it -- for the nymph's patience seems to be wearing thin!) but rather the violent action of the water.  The dangerousness and unpredictability of nature is what the nymph is calling attention to, not the scenes of near-perfect summer weather that the Shepherd seems to promise his lady-love.  The nymph is nothing if not careful and practical, and she uses this nature imagery to call attention to the impossibility of the Shepherd's promises. 

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.