I'm looking for the title/author of a book I read about 10 years ago: It's about a planet where all people who ever lived on earth are put after they died. They live alongside a very long river and are supplied with food from unknown sources. The main character heard a rumor that someone had wwoken up in a kind of storageroom before he was put on this planet and decides to follow the river upstream to find out who is putting humans on the planet. The book might be a lot older than ten years. I believe there exists a sequel, too. Please help me find the book. I'd really like to reread it!