The King of the Golden River is a fairy story; a book containing a copy of it was owned by Huey, Dewey and Louie Duck by 1958.
The story focuses on Gluck, a very unselfish boy. He came across a gnome who was in truth the King of the Golden River. Amazed by the selflessness Gluck displayed, the King taught him a "magic trick" which, if he performed it in front of a certain waterfall in a certain valley, would turn its waters into gold.
Gluck's Brothers, who were not as unselfish as him, learned of this and attempted to perform the trick themselves to become rich, only to be turned into smooth black stones instead in punishment by the magic of the Gnome King of the Golden River.
There may be a grain (or more than a grain) of truth to the story: in the late 1950's, while on a doctor-mandated vacation, Scrooge McDuck came across a Golden River almost identical to that in the tale, in a valley fitting the description of the valley in the story. By intervals, it became golden due to hot springs carrying gold dust up into the stream of water. Donald Duck and Huey, Dewey and Louie Duck thought to exploit this, playing tricks with Scrooge by posing as the King of the Golden River and thereby teaching him a lesson about his own selfishness. One doubtful account suggests that during these events, Dewey Duck briefly glimpsed the King of the Golden River himself, who had seemingly been orchestrating events.