Whenever the PCs come across an uncharted river, roll d20 on this chart (or d100 for a more mundane setting).
1: Holy. This river flows from a sacred spring and actually runs with holy water. It might be a magical forest brook or it might be a major trade river, with fervently religious cities on its bank, hordes of pilgrims rafting down its water, and holy-water bottlers and distributors driving ritualist clerics out of business.
2: Deep. Whatever its width, this river is so deep that it’s navigable by sea-going warships. In fact, its depth is unplumbed. It’s rumored to go all the way down to the Plane of Water. Every once in a while, bizarre sea monsters surface, snatch a ship, and disappear.
3: Magical border. One side of the river is reasonably civilized, with the occasional village and farmhouse along its bank. The other is monster-haunted wilderness, from which blood-curdling howls can be heard at night. How is it that such a long, meandering, un-patrolled border can hold back the forces of evil? It’s because these monsters can’t cross running water (they’re vampires, witches, or other such creatures) or because the other side of the river is the borderland of another plane (faerie or Ravenloft for instance).
4: Fertile. The farmland around this river is incredibly economically important. Like the Nile, it can support an unusually dense population and produce enough excess food to sustain the rest of the kingdom. Possession of such a river is worth warring for.
5: Not water. Blood, wine, mist, lava, mercury, acid, liquid sunlight, potion of delusion, stars, sighs, dreams – whatever it is, you can build a bridge over it and you might be able to sail on it (although you might need a magical boat).
6: Roll d6 twice more on this chart.
7+: Just a river.
Edit: Check out the comments for some more good ones!
A couple more?
Lethe: Forget stuff if you drink from it. Also maybe the river transports your memories downriver and fishermen listen to the fish whispering them.
Healing: Once per year you can get a Cure Serious, Cure Disease, or Neutralize Poison (whichever is most severely needed at the moment) by bathing in it. Water removed from it isn’t magical.
Hollow: The river flows atop a long pocket of air over a deeper riverbed. If you fall in and sink, you’ll fall out the bottom of the water and land on the moist ground. You can breathe just fine, but getting out is a challenge, and it’s never brighter than twilight. Cave elves hurl fishing lines upward and caravans of fish-men haul ocean goods up into the mountains.
Flaming: Pockets of gas erupt into flames with some regularity. Dancing lights can be seen at night. Ships sink suddenly into aerated water.
Uphill: River flows the wrong way. Ends at a mountain glacier where the water disperses and falls up into the clouds as reverse-rain.
Rejecting: A strange current shoves anything floating in the river off to the sides; impossible to travel up or down river, very difficult to get out into the middle, but once you’re there it spits you out to the other side.
Fishy: So many fish in the river that fishing is always successful. During spawning season, it’s possible to run across on the backs of fishes with a DEX check.
Poisoned: It flows from a dungeon and something in there makes the water bad. Crops dying, princesses lamenting, etc.
Animated: The river is one big elemental. Sometimes it rears up and strikes down a bridge or mill, which it considers offensive.
Wow.
I’m trying to imagine a world where every single river has some kind of quirk on par with these lists. It would actually be pretty frightening to encounter a new, unfamiliar river, and maps of the rivers (and guides with extensive knowledge of them) would be incredibly valuable. There might be a faction whose major mission is simply to create a complete catalog of the world’s rivers and their assorted properties.
In a game I ran set in Alexander the Great’s era, one of the reasons for the Persian Empire’s success was that they were adept at negotiating with, honoring, and placating river deities. With the rivers on your side, you are fertile in peace and treacherous in war. Until an actual son of Zeus shows up at the head of an invading Macedonian army and all the river deities break to his side, of course.
A river that is spawned from an opened decanter of endless water that simply needs to be closed. The farmers have come to rely on it, so claiming the object like most would as it is treasure will cause a food shortage and suffering to the locals.
A few more for the pot…
Necrotic: The water reeks of death and decay. Any water removed functions as unholy water, but any who enter the river itself will take ongoing unholy damage. Any corpses or bodily remains that are in the river will be raised as an undead within a minute and crawl back out, if they actually died within the river itself they will come back as a more powerful undead.
Mechanical: The river is actually a series of mechanical treadmills, the surfaces of which are covered with shifting paint to give the very real illusion of water and depth. Golem-like undines made of spinning gears and metal (that are similarly painted to appear life-like at a distance) use glass apparatus to mend any damage to the “river” and fend off aggressors. Under the “rivers” surface are hundreds of inter-connecting glass tubes and alchemical devices, seemingly transmuting a single fluid from the source of the river down its entire length via countless subtle alterations. The undines remain silent as to the purpose of their toil, but may disclose some secrets of alchemy, machinery or magical paints if appropriately engaged with.
Shrinking: The rivers water causes anything within it or on its surface to shrink in size so long as its in contact with the water, making the river seem more like a sea if anyone attempts to swim or raft across it! (Boats will have their entire contents and crew shrink with them). Water taken from the river will keep this property, but the containers hold it end up being very small. The amount of shrink can vary, some rivers continue to shrink objects over time, until they disappear, others are dependant on the width or depth of the river the water is taken from, whilst others still have a fixed shrink that is proportional to the length of the river.
Fortean: The river occasionally decides to rain its entire contents up into the sky and into a river shaped storm cloud which will float off appropriately with the winds (this including fish, frogs and any boats of swimmers who were in or on the river at this point, leaving them stuck in the cloud to find them own way back down, which is automatic when it starts to rain). The river is then replenished with a new river (roll on the chart again to find out what sort, in addition to the Fortean type) for a random length of time, before repeating the above process all over again (which can be even more wild depending on whatever the new river is).
Time: Crossing this river by bridge or jumping will rapidly age you, as will travelling down stream quicker than the current is flowing. Travelling along with the current will halt ageing. Travelling up stream will make you more youthful, but is very difficult to do in a boat as it too will become “youthful” (so you’ll need a very old boat if you want to make any lengthy journey without the boat falling apart!).
The mechanical one is really good. The time one is great because I’m imagining an industry of boat-ageing, ferrymen working dangerously because they whisk from ancient to youth every trip, etc.
Interplanar: Traveling on the river results in encounters that, if handled the right way, lead the traveler to emerge onto an identical river in a different plane. Maybe going upriver takes you to Mount Celestia and going downriver takes you to the Abyss. If you handle the encounters wrong (all very symbolic, but possible to do it right by chance or guesswork) you don’t change planes. Certain creatures (a troll bridge, a village, a fisherman) will be present in all three planes but different, so they aren’t directly connected.
Seven Voyages: Paddling into the full moon’s reflection on the river transports your boat to the next river in a sequence, possibly across the world. Because you can’t transport again immediately, it will take another 28×7 days before you can get back to your home river. A bit like the Sliders show. When you shift rivers it looks like you’re falling into a silvery-white tunnel as if there were a deep hole in the river, and you plop out onto the next river. Maybe roll for a boat handling skill to see if cargo or passengers get spilled, but they all end up in the same river so it’s more for a little immediate chaos.
Scroll: The river is made of shreds of parchment, paper, vellum, etc. Ribbons flow by and you can be splashed by a confetti of chads. The river is otherwise dry. Inkfish leap from scroll to scroll and if caught their black flesh tastes unhealthy and metallic. The tributaries are individual scribes’ wastebins if you follow them upriver far enough, and the river delta becomes aisles of a library. Librarians hurl strangely-knotted nets to capture the right information. Fishermen sometimes haul up entire antique tomes. The languages, styles, and completeness are entirely random.
Gravel: Bouncing boulders and pebbles fill the riverbed. Travel alongside it is dangerous and those who ride in metal barrels sometimes find it lethal. Upriver it’s mostly large stones that can be dodged with good rolls, but at the estuary they’ve crashed together enough that al have become smooth pebbles and stones. Great dust clouds rise from the grinding downriver. The ocean it pours into is a slurry of wet gravel for a long way out, and the gravel calms down by the time it spreads out and pours off the continental shelf.
Leaping: The river drops into sudden waterfalls pouring into deep caves, travels underground for a while, then launches upward to the surface again through another cave. These can be 90-degree shifts, or more gentle and navigable slopes.
Circular: The river just keeps going around. It’s an ancient snub against the Sea God, depriving him of its waters. But he doesn’t want it anyway. Really. Humph!
Tam: Girls born along the river often grow up strange and violent. They typically run off and join cults and pirate ships and circuses – and adventuring parties. Expectant mothers are reluctant to drink the waters and journey away from the river to give birth (at a time when journeys are difficult!) but it’s hard to tell how much this helps.
A friend and I had the beginnings of an adventure planned around a theme of grief and sadness, and we had a river in mind for it. We even had a scale of grief effects, similar to the existing fear rules.
Lachrymose: Fed by a spring at the feet of a weeping statue (a grieving widow turned to stone, left to weep forever), the water of this river causes debilitating sadness if consumed or bathed in (Will save to resist). The further up the river you go, the stronger the effect gets, with signs of previous adventurers and other sentient beings stumbling upon the river and simply… giving up at the banks of the river.