The Cocytos river can only be found in the Dreamy side, although its valley is present in the Awake Side.
The most defining characteristic of Cocytos are its cold waters. It is actually below the freezing point of pure water by 10 degrees, but doesn't freeze thanks to a salt called cocytide. This same salt is also responsible for its coldness.
Algae in the Cocytos is mostly limited to small growths less likely to freeze. It incorporates the cocytide salt within it. It serves as food for the fauna, notably composed of iceprawns and sturgeons.