The Mucky Mammoth travels down the Echo River in Act IV's overworld.

The Echo River is the setting for Act IV, during which the characters travel down the river stopping at a series of destinations along the way. Stops along the river include the Bureau of Reclaimed Spaces, a gas station, the Rum Colony, a telephone station, the Radvansky Center, an island, the Echo River Central Exchange, Sam and Ida's restaurant, and the Silo of Late Reflections. There are also a number of other people traveling down the Echo River that the characters meet; the Iron Pariah can also be encountered along the Echo. 

The river flows from Lake Lethe, which is named for a river in Ancient Greek mythology that erases people's memories so that they can travel on into the afterlife peacefully. Will remarks that the river has a way of making one forgetful. 

The Echo River is also largely the focus of the interlude Here And There Along The Echo.