While Percy does interact with river gods in the books, it's not in the context of stopping armies. In *The Sea of Monsters*, he does interact with the god of the East River (who is not named), but it's in a more personal, individual context.
Here's why this situation is unlikely:
* River gods' power: River gods have dominion over their specific waterways. They wouldn't necessarily be compelled to intervene in wars or battles happening on land, even if those battles affected the land surrounding their rivers.
* Percy's influence: Percy is a powerful demigod, but he wouldn't have the authority to command gods, particularly ancient and powerful river gods.
Let me know if you have any other questions about Percy Jackson or the *Percy Jackson & The Olympians* series.