Why is it called "levanon"? Because it "whitens" ("malbin") the sins of Israel, viz. (Ibid 1:18) "If your sins are like scarlet, they will become white as snow." (Devarim, Ibid.)
"until the great river, the river Perat": (so-called) because its greater part abuts on Eretz Yisrael, as in the folk homilies "The king's servant is likened to the king," "Rub shoulders with the anointed one and they will bow down to you too."
"the river Perat": (so-called) because in its beginning it can be dug up with a shovel, and it branches out ("mafreh" [like "Perat"]) until it must be crossed by boats. Variantly: (so-called) because vegetation "multiplies" (parin veravin) through it. All the rivers say to Perat: Why don't you let yourself be heard, just as we let ourselves be heard from afar? It replies: My deeds make me known. If something is sown upon me, it sprouts in three days; if something is planted upon me, it rises in three days. And thus does Scripture praise me: "until the great river, the river Perat."