Kimihito mentions about the Sanzu River when he's experiencing the near-death experience.
It is believed to be the boundary between the realms of the living and the dead, the dead must cross the river on their way to the realm known as Higan; an endless flower garden where there are no nights or days and no seasons, just a gentle warm light that covers the place as the dead await judgement on whether they will enter the Netherworld (following which they will eventually be re-encarnated and return to the world of the living) or Hell (where they will be doomed to remain for eternity).
The cover of this chapter resembles the opening scene of the anime.
In the second segment of the fourth episode of the anime Centaur no Nayami, the character Nozomi Gokuraku reads manga instead of studying. The book she holds shows the fourth and fifth pages of chapter 30 of Monster Musume.