In just the opening five words to Part III, “The Fire Sermon,” the speaker shatters at once our natural inclination to romanticize waterways as self-sustaining and enduring ecosystems.
(The River Thames has a rich history of being depicted this way in literature. For example, in the lines that end this portion Eliot recites the line “Sweet Thames, run softly till I end my song” from “Prothalamion,” by Edmund Spenser; another example that comes to mind is Conrad’s Heart of Darkness, in which Marlow finds great peace and serenity while on the same river.)

With bitter and mournful nostalgia, such portrayals of the river are juxtaposed, as we are given a dose of sad reality when reminded of the river’s impermanence: for in “tent” we think of portable and temporary shelter or canopy. With “broken,” we gain an understanding that the once brimming with life ecosystem in and around the river is now at death’s door, that life is not unfolding as it should and as it once did. Life itself, not just the “last finger of leaf,” is wilting. In this way, the river will soon be just another causality of “The Wasteland.”

What follows in “the last fingers of leaf / Clutch and sink into the wet bank” further suggests that these are the last vestiges of green, lush, leafy life. With the sense that the canopy is fading fast overhead, we then zoom out to see an even more barren and lifeless landscape then first imagined: “The wind / Crosses the brown land, unheard. The nymphs are departed.” This paints a picture of a cold and desolate place with little to no sign of life: there is no-one to hear the wind, not even the nymphs. Young, beautiful, and full of life, the departure of these divine spirits suggests that the magic of past literary eras has been forced out as well. However, perhaps this truth is intimated as apropos for a world post WWI -- the ornate literary conventions of the past will no longer suffice for a world in tatters. Such literary traditions will fail to get at the bitter truths. In another context in another poem, “departed” may entail a leave of absence. But not here — the nymphs are dead, rotting away in “The Wasteland.”
