Xanthus, of all the river gods, which I talked briefly about in my previous PBP post, is the most fascinating to me. This is the god of the sacred river in Lycia, though in the Iliad it seem to be featured closer to Ilium as to take part in defending Troy and especially active in the battle of the gods in this epic poem. This is alluded to because Hera has Hephaistos use his flames against the waters of Xanthus to boil away the water which causes enough distress that Xanthus finally gives way. The Iliad statues that Xanthus was the divine name of the river given by the gods, but that the river itself was called by mortal Scamander. That said, in myth, Xanthus gets his acclaim from the myth of Leto in which the goddess, after birthing the twins in Delos, is said to have traveled to Lycia, to the Xanthus river, where she bathed Apollon and the holy water was consecrated to him. Apollon is not the first god to have a river made sacred from the act of bathing the infant god, this is also the case of Zeus following his birth.
The reason Xanthus was chosen to be the bathing place for the infant was quite probably for a couple of different reasons. For one, of all the river gods this is the only one which is attributed as the son of Zeus and as such made it perhaps of greater prestige on that factor alone that Apollon be bathed in the greatest of rivers. It may also be because of the curious nature ascribed to the river in which it possessed two springs. One which issued cold water, and the other that issued forth hot water. It is said that was near this river that an oracle of Apollon was built to which, according to Delian legend, Apollon spent half of the year when not at Delos and so there had an ancient oracle. Therefore, it could be said that Xanthus perhaps played a part in the mysteries associated with Apollon in Lycia as Peneios played at Delphi as a purifying agent associated with the oracle. That it has a yellow color, which the name Xanthos also refers to, may have associated the river with the light of the sun. That another myth apparently ascribes the creation of the river to the birth pangs of Leto may suggest that Xanthos may have emerged as amniotic fluids that had surrounded the twins in the womb, and therefore would also have made Xanthos the first water to touch the twins and logically in this sense would have attributed him as the son of Zeus even as Hesiod names the river, as all rivers, the offspring of Okeanos and Tethys. It is of no doubt that Leto was particularly paid great honor is Asia Minor, where she was conceived as a great goddess with powers over the underworld in Ionia, and the Letoon, is a temple specific to Leto of which I have read of no other instances where she is given her own temple aside from that at Delos. This may then be an idea in which Delos and Lycia are both considered birth places of Apollon, one that was perhaps recognized by Delians in their relationship with the city.
Likewise the Old man of Lycia who returned Apollon to Delos and accompanied Apollon to Delphi to construct Apollon’s temple there, was said to have brought him back from the side of Xanthus to Delos initially. Therefore establishing two parallel traditions: One in which the Apollon is brought from Lycia, and its Hellenic interpretation that the god was returned to Delos from Lycia. I believe, if my memory serves me, that he was the first to discover the god at the side of the river and is therefore associated as the most ancient seer associated with his cult. The maidens of Delos in fact had a song that they specifically sang in his honor during their performance in the festivals.