The theme is the message that an author is trying to get his or her readers to understand. For example, in the Greek myth of Daedalus and Icarus, the theme was "don't be prideful". Because Icarus and Daedalus violated this by trying to act like the gods, they were punished: Icarus fell into the ocean and drowned.
Answer:
The poet refers long lasting love or a strong love connection to rivers that cannot quench. It is so great that not even a river can "quench" it, and the only thing that can give her "recompense" is love from her husband. The word "quench" means lots of things, which makes it tough to suss out the meaning of this line. It usually means to extinguish, put out, or satisfy.