For "The Fire Sermon", and the associated imagery of the normally trash laden and polluted Thames. Eliot muses of earlier times with his father fishing, only interrupted by one solitary rat (cue "Wharf Rat" by the Grateful Dead...), so it would seem as this were an idealized version of memory. There is also reference to "the river Leman", but it does not meet the regular word meaning. Feels more like the river in Dante's Purgatory, or perhaps in the Klingon ritual of Gre'thor, by riding a barge on the river Skral. The traditional meaning of "sweetheart" does not seem to apply here.