Every Greek river had a spirit; in the Illiad the spirit of a nearby river
rises up and threatens Achilles for dumping so many bodies into him. And
a great many such river-dieties had daughters - nymphs. Daphne is the
most famous; Apollo pursued her, and she called out to her father for
salvation, causing him to turn her into a laurel tree. Apollo made a
wreath of her branches and wore it thereafter, which is why champions in games
dedicated to Apollo were given crowns of Laurel, and why we speak of honors as
'laurels' today.
This is of course Greek and not Tolkien's favorite
northwestern mythology, but Rhinemaidens and such are common as
dirt.
