The River of Junk is a geographical feature of the Land of Ooo seen in the episode "The Witch's Garden." Gary the Mermaid Queen currently lives in the river and her hatchlings live in the tree right next to the river.
In the episode Finn and Jake have to travel across the River of Junk because there is a mud puddle Jake thinks may restore his magical powers, which were stolen by the Witch. However, Jake does not want to cross because he thinks it would be too hard for him. Finn manages to get him across the river and into the mud puddle by building a catapult made of driftwood and a trash can lid. When he and Jake reach the mud puddle, they discover that it is not the right mud puddle.
The River of Junk is seen in a sandy place filled with the ruins of old buildings, planes, and boats. It is filled with bags of garbage and other seemingly random, broken objects. On the other side, a mud puddle is situated.