Achelous was prepared to marry his soon-to-be wife Deianira, the Princess of Clydon, until Hercules saw her, and challenged Achelous for Deianira. During this fight, Hercules cut off Achelous' horn and used this as a way to humiliate him; the horn later turned into a cornucopia. Deianira picked Hercules over Achelous, leaving Achelous bitter and jealous of the strong hero.
Jason Grace and Piper McLean are sent on a quest by Hercules to retrieve Achelous' other horn to humiliate him even further (Hercules had cut off his first one). When Jason and Piper get to his river, Achelous is singing. Piper thinks that the singing has the same effects as charmspeaking. Achelous assumes that the demigods are there to kill him at the request of Hercules. He explains to them why Hercules did not deserve his horn and tells them the story of Deianira. Although both Piper and Jason feel bad for Achelous, who seems to be friendly, the god tries to stop them from leaving by attempting to drown them. Whilst Jason is pulled under, Piper tries to bargain with Achelous, but the god believes that she wants to be his bride, to compensate for Deianira. When Achelous picks her up, she threatens him with her with her knife to let Jason go. Piper then cuts off the god's horn and leaps for the shore, using Jason's power over the winds to carry her safely to the bank. Achelous wails as he sinks back into the river to mourn the loss of his horn.
Hercules never gets Achelous' horn because Piper forces him to give them permission for their ship to be able to leave. Piper uses the horn as a cornucopia on Hercules, and food comes pouring out of it, burying the god. The Argo II flees from Hercules' island, leaving behind a mourning Achelous and an angry Hercules.
Piper sees a vision in Katoptris of Achelous begging her for his missing horn back.
Achelous is a bull with a human face, short curly black hair, and a ringlet beard in the style of Ancient Greece. His skin is blue like his river, eyes that seem deep and mournful behind bifocal glasses, and a pouting mouth. He has a single, curved, black-and-white bull's horn that sprouts from the left side of his head though that was later cut off by Piper.
He has the standard powers of a god.
- Hydrokinesis: As a river god, he has great control over water, mainly his river.