The river's daughter (presumably the river Withywindle) and Tom Bombadil's merry and beautiful wife in Tolkien's The Lord of the Rings.

"O slender as a willow-wand! O clearer than clear water!
O reed by the living pool! Fair River-Daughter!
O spring-time and summer-time, and spring again after!
O wind on the waterfall, and the leaves' laughter!"
-(from The Fellowship of the Ring)

I'm struck by the eerie similarity between her and Polychrome, the rainbow's daughter, from the Oz books. The ethereal beauty, the diet of dew drops, the whole water-daughter thing.. of course, even in her bit-part, Goldberry carries a lot more dignity than Polychrome ever did, P's charming sweetness aside.