Others derived the legends about Achelous from Egypt, and describe him as a second Nilus. But however this may be, he was from the earliest times considered to be a great divinity throughout Greece, and was invoked in prayers, sacrifices, on taking oaths, &c., and the Dodonean Zeus usually added to each oracle he gave, the command to offer sacrifices to Achelous. This wide extent of the worship of Achelous also accounts for his being regarded as the representative of sweet water in general, that is, as the source of all nourishment.
The contest of Achelous with Heracles was represented on the throne of Amyclae, and in the treasury of the Megarians at Olympia there was a statue of him made by Dontas of cedarwood and gold. On several coins of Acarnania the god is represented as a bull with the head of an old man.
The sacred bull the serpent and the Minotaur are all creatures associated with the Earth goddess Gaia. Achelous was most often depicted as a gray-haired old man or a vigorous bearded man in his prime, with a horned head and a serpent-like body. When he battled Heracles over the river nymph Deianeira, Achelous turned himself into a bull. Heracles tore off one of his horns and forced the god to surrender. Achelous had to trade the goat horn of Amalthea to get it back. Heracles gave it to the Naiads, who transformed it into the cornucopia. Achelous relates the bitter episode afterwards to Theseus in Ovid's Metamorphoses. Sophocles makes Deianeira relate these occurrences in a somewhat different manner.
The mouth of the Achelous river was the spot where Alcmaeon finally found peace from the Erinyes. Achelous offered him Callirhoe, his daughter, in marriage if Alcmaeon would retrieve the clothing and jewelry his mother Eriphyle had been wearing when she sent her husband Amphiaraus to his death. Alcmaeon had to retrieve the clothes from King Phegeus, who sent his sons to kill Alcmaeon.
Ovid in his Metamorphoses provided a descriptive interlude when Theseus is the guest of Achelous, waiting for the river's raging flood to subside: "He entered the dark building, made of spongy pumice, and rough tuff. The floor was moist with soft moss, and the ceiling banded with freshwater mussel and oyster shells. In sixteenth-century Italy, an aspect of the revival of Antiquity was the desire to recreate Classical spaces as extensions of the revived villa. Ovid's description of the cave of Achelous provided some specific inspiration to patrons in France as well as Italy for the Mannerist garden grotto, with its cool dampness, tuff vaulting and shellwork walls. The banquet served by Ovid's Achelous offered a prototype for Italian midday feasts in the fountain-cooled shade of garden grottoes.
At the mouth of the Achelous River lie the Echinades Islands. According to Ovid's pretty myth-making in the same Metamorphoses episode, the Echinades Islands were once five nymphs. Unfortunately for them, they forgot to honor Achelous in their festivities, and the god was so angry about this slight that he turned them into the islands.
In Hellenistic and Roman contexts, the river god was often reduced to a mask and used decoratively as an emblem of water, "his uncut hair wreathed with reeds". The feature survived in Romanesque carved details and flowered duruing the Middle Ages as one of the Classical prototypes of the Green Man.
When Achelous on one occasion had lost his daughters, the Sirens, and in his grief invoked his mother Gaea, she received him to her bosom, and on the spot where she received him, she caused the river bearing his name to gush forth.
Other accounts about the origin of the river and its name are given by Stephanus of Byzantium, Strabo, and Plutarch. Strabo proposes a very ingenious interpretation of the legends about Achelous, all of which according to him arose from the nature of the river itself. It resembled a bull's voice in the noise of the water; its windings and its reaches gave rise to the story about his forming himself into a serpent and about his horns; the formation of islands at the mouth of the river requires no explanation. His conquest by Heracles lastly refers to the embankments by which Heracles confined the river to its bed and thus gained large tracts of land for cultivation, which are expressed by the horn of plenty.