They are the two rivers of the Earthly Paradise, which rise from a single source: Lethe takes from whoever drinks it the memory of sin, and Eunoe restores the memory of every good deed. The water of the second acts only if the water of the first has been tasted beforehand.
The one who explains it is Matelda, in canto XXVIII. She says that the water does not come from a vein that restores vapor turned to ice, like a river that gains and loses force, but from a firm and certain source, which draws from God's will as much as it pours out on both sides. Lethe appears earlier, in canto XXVI, as a measure of Guinizelli's affection for Dante: the trace Dante left in him not even Lethe could erase.
The immersion in Lethe happens in canto XXXI, after the confession: Matelda embraces Dante's head and submerges him, and he had to swallow the water. In canto XXXIII, Beatrice uses the river against him: if he does not remember having turned away from her, it is because he drank from Lethe, and forgetting proves guilt. Eunoe closes the cantica, with Dante renewed.
The order of the waters says something about purgation: first the forgetting of evil, then the remembrance of good. It does not end in the erased fault; it ends in the good that was done.