For Eliot, the Thames River is a place that's been immortalized by English poetry for centuries. But in the modern world, the Thames is just a filthy, polluted waterway whose banks are filled with litter and slimy rats (175-188). Yum.
Pollution's an image that comes up in other places in this poem, too, like with the "brown fog" that covers London in physical and spiritual dirt (208). Overall, the pollution represents the destruction of things that were once great. All of the objects that pollute the banks of the Thames are also disposable things that were brought in by modern culture, like sandwich papers, bottles, or cigarette butts. These items all leave traces of people who are drinking and smoking—rather than frolicking and marrying and poeting, like in ye olden times.
Lines 173-186: Eliot opens "The Fire Sermon" by painting a pretty dismal picture of London's Thames River. In line 176, he quotes the great English poet Edmund Spenser, a man who once wrote love songs about how beautiful and inspiring the "Sweet Thames" was. In modern days, though, Eliot only finds "empty bottles, sandwich papers, / Silk handkerchiefs, cardboard boxes, cigarette ends."
Yeah, we know—he actually says that "the river bears no
empty bottles, sandwich papers" (emphasis ours). But our man T.S. is
getting his sarcasm on. His Thames river is way filthier than Spenser's.
Did your mom ever walk into your bedroom and say "Wow. Sure is spotless
in here. You definitely shouldn't clean anything up"? Then you know exactly the tone that Eliot is adopting.
The polluting of a once-inspiring river is connected to the moral pollution that has affected a once-inspiring civilization. Maybe the most depressing image of this lost magic comes in Eliot's line, "The nymphs are departed." This line suggests not only that the Thames has lost its former beauty, but that the river has lost a sense of mythological awesomeness that it can never get back.
Lines 266-269: When these lines talk about how "The river sweats / Oil and tar," the description comes right on the heels of a beautiful images of "Ionian white and gold" (265). So basically Eliot's giving you a sense of how beautiful the world could be, then slapping you in the face with an image of how ugly it actually is. He also talks about the pollution of the river by slipping into the form of a popular song, drawing even more of a comparison between environmental filth and the moral pollution of pop culture.