In Garth Nix's Sabriel Trilogy, the River of Death is the realm the dead go through on their passage from life to being permanently, truly dead.
The river takes the form of an infinitely wide expanse of cold water, flowing downwards. Any spirit in the river feels a "physical" drag downwards, as well as mental compulsion to go downwards with the river, leaving life behind.
On its journey downward, the river passes through nine precints and nine gates. After passing through the ninth gate, the spirits of the dead are truly departed, and can never return to life. Each precint of death as a different look and feel to it, with some being extremely dangerous and hard to stay in, while others allow the spirits of the dead to tarry longer.
Many of the spirits of the dead can tarry in some of the precints for years or even centuries, struggling against the current and trying to feast on those still living in an attempt to have enough energy to cheat death. The longer they stay dead, the less of their human spirit they maintain. Many of the Dead are either in league with, or controlled by, Necromancers, who can walk in spirit forms through death.
There are two interesting points about the River of Death: first, even though the Dead live in a realm of metaphorical running water, dead spirits are afraid of and can be damaged by physical running water. What the connection between these two facts is, and whether Garth Nix gave it much thought, is unknown. The other facet of the River of Death is that it seems to be outside of Nix's cosmological scheme of Free Magic and Charter Magic. While Free Magic necromancers and Charter Magic Abhorsens both can work inside of the River, the River itself seems to be beyond the realm of either.