The Phlegethon river exists solely within the Dreamy Side. Its influence on the Awake Side has carved a valley, however.
Phlegethon, as all natural water sources, is filled with many impurities. Besides common sediments, it has phosphorus compounds in it that render it burning to the touch. Particularly, an alkali supersalt called phlegethontide. The water is constantly over 95ºC and it also sparkles.
Few are the animals and plants that can inhabit its burning waters, but there are exceptions. Fireprawns are crustaceans capable of bathing in those waters, and are known to have uncookable flesh. The feed mostly on zooplankton with the same features.
Phlegophyceans are algae native to the river, also imune to its heat. They are not even photosynthetic, actually, being thermosynthetic. This allows them to be light yellow, white or even transparent. They are a major food source for snails and fish that inhabit the waters.