Riverworld is a roughly Earth-sized planet whose major defining feature is a single wide Rivervalley, and where some 36 billion humans from many eras of civilization are resurrected long after their deaths. The Rivervalley follows a twisting path back and forth down one hemisphere of the planet and up the other, circling the south pole, and both originating and returning to a small sea at the north pole. The mountains that enclose the valley are higher than the Himalayas and consist of sheer basalt cliffs for much of their ascent. The valleys contain smooth grassy flatland close to the River, rising into low forested hills the closer one gets to the mountains. The plant life consists only of grass, bamboo, and a few varieties of trees, mainly oaks and pines. The only non-Terrestrial plant life seen are the massive Irontrees, and the flowered vines that grow on them.
The Valley averages several miles wide at most points, and the River is over a mile wide, sometimes widening further into lakes. It flows continuously, despite its source and end being on the same level, due to a hidden series of pumps under the Riverbed. The River contains many kinds of fish, including several large scaleless fish whose skin can be tanned into leather. The largest fish, the Riverdragon, is the most dangerous of all these species.
The humans resurrected along the Rivervalley awoke naked, in the prime of health, around 25 years or younger in physical age, and with all hair shorn off. Each had a band on their wrist connected to a hollow cylinder of strange metal which many came to call a Grail, among other names such as a "lunch pail" or "glory bucket". Only the person who was attached to a particular Grail can open that grail, with the exception of "free Grails" left as demonstrations for the newly resurrected.
Along the shores of the River, and at some points inland, are the structures which most Valleydwellers come to call Grailstones. When a Grail is placed on the Grailstone's surface, three times per day, there is a massive discharge of energy from the stone which leaves the Grail filled with food and sometimes other items such as firestarters and clothing.