River God

River Gods are powerful water spirits that are male in form when they are on land. They are basically the male equivalent of Naiads.

One River God is mentioned rising out of the Great River of Narnia with his Naiad daughters.

Other River Gods are mentioned as becoming husbands to the daughters of King Frank I and Queen Helen in The Magician's Nephew.

Another River God is mentioned lifting his head from the river with the river nymphs upon hearing Aslan's roar. He appears again, and asks Aslan to loose his chains in Prince Caspian.

Description
River Gods are basically described as looking like ordinary men in their physical forms, with some minor difference, such as blue skin, seaweed in their hair...etc.

Film
In the 2008 film Prince Caspian, the River God is digitally portrayed as a giant man made entirely out of water with hair and beard made of foam, and rushes at the Telmarines, creating tidal waves, and even destroying the bridge.