The Planes have many waterways, some more hostile than others. The names of Styx and Oceanus will be well-known to all planewalkers. But these two are far from being the only rivers in the Multiverse. Here I intend to discuss some other rivers: Eridanus, Pyriphlegethon, Lethe, Eunoë, and others.
Eridanus: The River Eridanus flows from Mount Celestia to join the Oceanus in Elysium. Known as the River of the Heavens, or the Astronomers' River, its surface sparkles with motes of light, and during the dark hours it can be seen many leagues away sparkling like many stars. Rising in Mertion at the small town of Alpha Eridani, it flows down the Mountain as far as Epsilon Eridani in Mercuria, where it crosses into Buxenus in Arcadia at one very obvious point. The first town it reaches in Arcadia is named Zeta Eridani, and it flows across the plane and into Abellio (against all intuition, this means it should be flowing uphill, but the topology of Arcadia is such that it always seems to find a downward slope to link two points). From Mu Eridani in Arcadia it flows to the Cordant Plane of K'un Lun, and thence into Amoria in Elysium, and then down to Eronia where it flows into Oceanus at a village called Omega Eridani. The river is inhabited by strange radiant beings that remind those primes who understand space travel of gonn and constellates, although smaller by far.
Pyriphlegethon: Flowing from Mount Erebus in Phlegethos, Baator, the River Pyriphlegethon changes in nature along its length. At its source it is pure molten rock, much like everything else in the layer. From there it loops around to Phlegethon in Pandemonium where it is a congealed, nearly solid flow of lava and hot mud. Mind you, it's still much hotter than the rest of the layer, so a body who needs heat badly could do worse than to go looking for the flaming river. Passing next into the uncharted depths of the Abyss, the river's path is lost. It emerges again in the Hinterlands, near Xiuhtecuhtli's realm. Passing by the Dark Forest encircling the Slavic Other World as a wide chasm of flames, it flows along the Long Valley as far as the eye can scan. Somehow it next flows into Avernus, where it is only distinguishable from the River of Blood by temperature: Pyriphlegethon boils. Having thus gone full circle around the Lower Planes it flows into the Styx at Stenching Evil, the City of Revulsion.
Lethe: The River Lethe is noted as flowing through Nessus, Baator, and Hades' Realm, Pluton, Grey Waste. In fact, the river is something of a mystery. It rises somewhere in Chronias, Mt. Celestia, alongside the Eunoë, it is told. It then flows down every layer of the Mountain, running in a broad, straight canal through Yetsirah, and thence wends its way down to the slopes of Lunia. However, it does not flow into the ocean there, but rather enters a cave and flows away into the depths of the Mountain, next appearing at the base of Mount Purgatory. Thence it vanishes, emerging in Nessus, of all places. Quite how Purgatory is linked to Nessus is unclear, because the Nessus end of the river is always guarded by fiends. Some say that the enigmatic Lord of the Ninth seals it himself from time to time. Having thus entered Nessus, it flows parallel to the Styx into Pluton, where it eventually joins it in Hades' Realm. Lethe is called the River of Forgetfulness: Whoever drinks it forgets everything, just as with the Styx. However, unlike the Styx there is a way of retrieving lost memories from Lethe: simply walk upstream one mile for every year of life forgotten.
Eunoë: The River of Remembrance, Eunoë runs alongside the Lethe from their joint source in Chronias as far as Lunia. There Lethe goes underground and Eunoë joins the ocean. Eunoë appears similar to Lethe, but has a miraculously different property. Whoever drinks the water of the River of Remembrance regains memories lost by magic or psionics or by drinking the waters of the Styx or Lethe. However, traumatic experiences are rendered less awful by the process, and so many use it as a means of spiritual refreshment. Of course, to partake of it you must first be able to travel around the Holy Mountain, which prevents evil or overly chaotic berks from using it. (The effects resemble those of the spell blessed remembrance)
Cocytus: The River of Wailing, the Frozen River, the Fourth River of Hell. All names for one of the strangest rivers in the planes. Why so strange? Because it's frozen along its whole length, berk. It's pointless to talk about the rise and flow of this one, because there isn't any. But there it is: In Cocytus, Pandemonium, and in Cania, Nessus and Stygia, Baator. Walking along the ice may lead you to a portal linking one frozen stretch to another. The river fills all of Cania, and lurks in the crevasses in Cocytus and Nessus. It 'flows into' the Styx in Stygia, although that's an economy of the truth when talking about something which hasn't remotely flowed for aeons. Rumours tell of ancient fiends and other things trapped in the ice, as well as luckless petitioners who angered the Lords of the Nine. One rumour even tells of a former Lord of the Ninth (also a fallen archon, the story goes) frozen in a deep canyon in Nessus.
Acheron: Told of mainly in legend, the River Acheron is said to flow across the bottom of Ocanthus, in Acheron. Allegedly made of tar with iron plates floating in it, the Acheron is inhospitable and vile. Symbolically speaking, it's the entrance to the Lower Planes, as many sages claim that Acheron moves away from Evil and towards Law as you descend the layers, and so Ocanthus must be the closest to Mechanus, they say. Acheron is also the home to Charon, the Boatman of the Lower Planes. This enigmatic demipower is rumoured to be an ancient yugoloth of unknown race who commands, or possibly trains, the marraenoloths in the art of pragmatism and exploitation. His representative in the wider area of the Lower Planes is an ultroloth who looks like a large marraenoloth and who calls himself Phlegyas. Be very careful when dealing with either of them.
Achelous: A tributary of the Oceanus, the Achelous rises somewhere beyond Arvandor and flows across the first layer of Arborea to meet Ocean at the foot of Olympus. The river passes through acres of fine farmland, and the locals swear that it is the river that provides all that they need. The Achelous is guarded by a fierce power also called Achelous, who can shapechange and frequently appears as an enormous river serpent, or as a bull. Heracles has fought with Achelous at least twice, and the river god is still smarting from the last time and wants a rematch.