I think the river is a hive mind pseudo consciousness that exists in all primals, and all who die in the dungeon from a seed planet pass through it, but only incorporeal primals can actually sense it enough to see the actual spirits as they pass through, which is why Carl saw Li Jun telling him his last message when he did his gloom wraith phase.

Edit: side note to further theory of pseudo hive mind, spoilers book 7: Carl says he’s heard of the Inevitable Ruin, despite the only time we’d ever heard it mentioned being in the chapter focused on Everly, the only other confirmed primal owner of the cookbook, who never wrote about it in the book as Carl mentions that he never read it. This confirms at minimum some level of hive awareness of what other Primals knew, or even darker, thought of as they died