Fallen Creek does not appear in story mode. It is available as a stage in Showdown Mode. The connection between this location and the discovery of gold is established by the store description for the Gold Nugget, which unlocks the showdown stage: "Nate Harlow's first nugget."
The location is a snowy, wooded mountaintop. A creek runs along the lower area of the map, passing through a pine forest. Up the slope there is a ruined wooden cabin and a collection of large rocks.
The player can unlock this location as a stage in Showdown Mode by purchasing the Gold Nugget from the Shop interface at the start of the mission "Fort Diego". The subtitle for this location is "Snow-Topped Mountain."
Sheriff Bartlett's Journal
This location does not appear in Sheriff Bartlett's Journal.