Sweet Thames, run softly, till I end my song.
The inclusion of Prothalamion especially after the ending of A Chess Game, is a very conscious choice that Eliot made. The imagery of "the river's tent is broken: the last finger of leaf / Clutch and sink into the wet bank" alludes to Ophelia's suicide due to her grief, as well as Hamlet's choice not to marry her but to keep her around. The Prothalamion is a marriage song, yet it is based on a story of sexual assault, with Jove luring Leda as a swan and then raping her. This continues the note that A Chess Game left - marriage/love being a woman's downfall, as well as it also being the only reason women exist.
The contrast between the beauty and melancholy of "Sweet Thames, run softly, till I end my song" and the brutality of the origin of the song further emphasizes the cycle of beauty existing within death and carnage.