The poem "The Nymph's Reply to the Shepherd", also called "Answer to Marlowe", is a direct reply to Christopher Marlowe's earlier poem "The Passionate Shepherd To His Love." This satirical poem is meant, mostly tongue-in-cheek, to answer image-by-image and promise-by-promise the fanciful things the Shepherd of Marlowe's poem 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.