How to use a river god

Returning to Athens from the hunt in Calydon, Theseus and his companions are warned not to attempt to cross the river Achelous. By Achelous.

Theseus visits Achelous

". . . do not commit yourself to my devouring waters. They are liable to carry solid tree-trunks along, in their roaring, and roll great boulders over on their sides. I have seen whole stables, near the bank, swept away, with all their livestock: and neither the cattle’s strength nor the horses’ speed was of any use. Many a strong man has been lost in the whirling vortices, when the torrent was loosed, after mountain snows. You will be safer to stay till my river runs in its normal channel, when its bed holds only a slender stream."

As Prof. Anderson notes, there's an interesting split here between the voice of the river and the rapacious waters it speaks of. Achelous expresses genuine concern for Theseus, but the root of the concern is about the superhuman force of his own waters. He speaks of his power as though it belonged to another. When humans do this, it often has a comical quality, because of the suggestion of compulsion -- e.g., a boxer who cannot stop throwing punches continually has to warn people to beware of his fists. The god asks the Athenian hero, whom he admires, to pause and use his hospitality rather than hubristically dare to cross at this time.

With a witty zeugma, Theseus agrees to use both Achelous' home and his counsel:

Adnuit Aegides, “utar” que “Acheloe, domoque
consilioque tuo” respondit; et usus utroque est.

Hercules and Achelous

The warning of Achelous speaks to the question of scale. For the ancients, the gods were powerful and immortal, but still capable of being imaginatively represented (as opposed to the god of the Old Testament, who forbids any attempt to depict him, yet nonetheless is anthropomorphized within certain kinds of stories). A river god is a mysterious flowing presence -- rivers are far better known for their endings than their often veiled beginnings -- and they possess powers to fertilize, nourish and destroy. A river seems not to be able to go from flood to calm at will, though as Achelous will go on to say, he can change into a bull and a serpent, as he did when he wrestled Hercules. (Similarly, Achilles in the Trojan war will fight Xanthus, aka Scamander.)

Hesiod conveys something of the fertile variety of rivers in his catalog in the Theogony. The power to remember the names of all the Earth's streams is beyond any mortal:

And Tethys bore to Ocean eddying rivers, Nilus, and Alpheus, and deep-swirling Eridanus, Strymon, and Maeander, and the fair stream of Ister, and Phasis, and Rhesus, and the silver eddies of Achelous, Nessus, and Rhodius, Haliacmon, and Heptaporus, Granicus, and Aesepus, and holy Simois, and Peneus, and Hermus, and Caicus' fair stream, and great Sangarius, Ladon, Parthenius, Euenus, Ardescus, and divine Scamander. Also she brought forth a holy company of daughters1who with the lord Apollo and the Rivers have youths in their keeping—to this charge Zeus appointed them—Peitho, and Admete, and Ianthe, and Electra, and Doris, and Prymno, and Urania divine in form, Hippo, Clymene, Rhodea, and Callirrhoe, Zeuxo and Clytie, and Idyia, and Pasithoe, Plexaura, and Galaxaura, and lovely Dione, Melobosis and Thoe and handsome Polydora, Cerceis lovely of form, and soft eyed Pluto, Perseis, Ianeira, Acaste, Xanthe, Petraea the fair, Menestho, and Europa, Metis, and Eurynome, and Telesto saffron-clad, Chryseis and Asia and charming Calypso, Eudora, and Tyche, Amphirho, and Ocyrrhoe, and Styx who is the chiefest of them all. These are the eldest daughters that sprang from Ocean and Tethys; but there are many besides. For there are three thousand neat-ankled daughters of Ocean who are dispersed far and wide, and in every place alike serve the earth and the deep waters, children who are glorious among goddesses. And as many other rivers are there, babbling as they flow, sons of Ocean, whom queenly Tethys bare, but their names it is hard for a mortal man to tell, but people know those by which they severally dwell. Theogony 337 ff

Parada also has an annotated list of River Gods.