Jason lunged toward him, summoning the winds to lift him out of the river, but Achelous was quicker and more powerful. A curl of water slammed into Jason and sent him under once more.
“Stop it!” Piper screamed.
Using charmspeak wasn’t easy when she was floundering in a whirlpool, but she got Achelous’s attention.
“I’m afraid I can’t stop,” said the river god. “I can’t let Hercules have my other horn. It would be mortifying.”

The Mark of Athena, Chapter XXVIII