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.