The valley of the green river was a location on the planet Maltos IV that hosted a group of native peoples.
The group of natives in the valley of the green river encountered Captain Jonathan Archer. Later, they were conquered by Klingon warriors and subjugated as slaves. James T. Kirk would then arrive and lead the liberation of the native people in this valley, who would write the Book of Fulfillment about the events of their liberation. (TOS - Strange New Worlds 9 short story: "Book of Fulfillment")