Rivers of Lava

Yeah, it would be, but I'd like at least one crossing point for each river too if at all possible.

I really don't want a magical answer, though, anyone could just explain it away like that. I want what most folks are magically capable of to be fairly limited while living in a world with crazy shit like this. The world is high magic, they are not.

Well then you're going to be dealing with mafic lava which erupts at temperatures around 1,740 °F (950 °C) which gives it a very low viscosity. Now this is probably want you want since it'll flow like an actual river but you only get this type of lava in oceanic crust rather than continental; of course you can always just invoke magic waive that problem without losing much of anything. While hot as fuck it's not like you would be unable to approach the river though, so long as you don't plan to stay long you could get within five yards safely.

Building a bridge over it would be the only choice to cross over it easily. But it's not like there aren't materials that can't handle that amount of heat: aluminum oxide, cast iron, graphite, tungsten, and similar materials can handle it. You'd need a pretty wide bridge through to make sure the temperature on top of it is tolerable.

hot air balloons?

Couldn't you just build a huge tower either side with a bridge across it?
I think you could put supports diagonally between them if you want but is the architecture really more important than the concept you want?

Then it's ancient magic that no one understands how to do any more, and they have to pray that it doesn't stop working or they're screwed. If it's like the Tigris and Euphrates then your !Babylonians or !Sumerians could have built it in ages past.

Looong time ago, the hero Jenicenna trekked through the burning lands, and came across the molten rivers. Finding some people that wished to cross, he set about quarrying stone with his bare fists. He then found a mage, expert in enchanting stones, and told him to enchant the stone pillars he made. After the pillars got inscribed with the ancient magics to withstand the heat of the molten rivers, he launched them right into the middle of the rivers, forming a row of pillars. Then he quarried more stone slabs, and punched them together so they form long slabs. These he stacked, until a stone bridge safe to cross was built. Since then, the bridge has been maintained by the descendants of the people there.

"Hey Veeky Forums, what's a way for people to cross a river of lava without magic in a fantasy setting?"
>Magic
"No but seriously, how do they do it without magic?"
>Ancient magic
"The world is magic, they really aren't. They can't use magic to cross the river, only some mundane process. Can you think of anything?"
>So this guy finds a mage.....

I would write up some nonsense about how a wizard engineered a magic bridge some centuries ago for the locals. The spell he used draws energy from the volcanoes themselves to preserve it and keep the bridge and its passengers protected from the heat.

Cast iron would be possible, kinda like the Iron Bridger in England? I'm concerned about the brittleness of cast iron for something like this but if that fucker's still standing I guess it's strong enough. How wide do you think it'd need to be, comparable to a modern 4 lane bridge?

There's a canal to channel water from a nearby river and fill a "crossing pond". When the pond is full, they pour water at a specific spot in the river where a small lava dam is built. This blocks the lava flow for about 1 hour, then the dam opens and drains. During this hour window, they release the pond through a canal below the dam, forming a pool of boiling water. They use covered raft (pulled across the pond by rope) to quickly ferry goods and people across the boiling canal.

In the rainy months, the rivers are crossed once a week, but in the drier months, there is usually not enough water, so people find alternative routes to avoid the rivers. There is a big fedtival held every year to celebrate the first crossing of the rainy season, and it features busy markets on either side of the river, each set up to trade rare delicacies and goods from the other side.

If the land-gain into the sea is fairly minimal, it makes sense to have a couple boats regularly ferry people via ship around the lava-rivers.

Not through the lava-rivers, that would be warm. Around.

Alternately, you could have people try to cut tunnels into the land such that the lava-rivers are now underground, and block off the aboveground parts. Sure, you'll likely lose people doing so, but they're serfs or slaves, nobody cares.

Alternately, you could have some strange fucker with a proclivity for odd chemicals develop primitive hot-air balloons or some weird parachute-basket things. Hook them onto ropes/cables stretched over the river and incorporate ways to vent the heat and you're good.

Or just send a party to the source of the lava rivers, have them stab whoever is causing this until they stop.

Or just magic it. It's a weird enough environs to attract wandering wizards.

If your problem is rivers of lava, the solution is simple:
A river of ice.

Dig alternate lava route with premade stone bridge/it goes into a manmade tunnel.
Fill in lava rivers current position to force into new route with magma tolerant rocks.
Every so often have to make new crossing as old tunnel clogs up, if tunnel idea used.

Alternative: combat the heat by having the bridge be an aquaduct water channel. Run the water along fast enough and it wont have time to heat up to deadly amounts and is safely transversable, it a bit strong current.

Done.

Alright, got an update on the setting for you guys.

The large peninsula of Maron sets on against the western sea and thereby the central western edge of the known world. The twin inland volcanoes of Net and Ket in the Great Red Range of mountains continually pump out lava that flows through this peninsula to the coast. It reaches the coast and both rivers form into the Fire Delta that pours into the sea, crumbling and sinking into the heavy and strange waters of that terrible body. Huge clouds of steam and smoke are released from this which drift on the winds to and fro, inland and out to sea, making this area a haven for the roughest of brigands and especially pirates. These pirates use the smoke and hot fog to ambush many ships trying to sail around the tip of Maron to avoid crossing the great burning rivers. Eventually this hot and dirty smog rises and rains down in an acidic and grimy rain upon the inland area.

Thus getting around the rivers is quite difficult and forces the inhabitants to instead go over them. Personal travel and very light cargo transport is done via gondola that travels over the rivers, connected by to large towers on opposite banks. For more serious cargo there's only one option; the Wolfram Bridges.

Named for the dull grey metal of its construction, the Wolfram Bridges span each of the glowing rivers at great height with an incredible, but mostly unused, width. Almost certainly built by one of the advanced civilizations from before the Entai Sunderings, the bridges have stood for for as long as any of the inhabitants of the area can recall and are mentioned completely through recorded texts right up to the Death of History of the area, all but confirming this great age. The top road layer is comprised of gravel that protects crossers from the intense heat of the river below and the metal of the bridge around them. This gravel must be replaced routines and ploughed several times daily to avoid it solidifying into a solid mass in the heat.

The road is quite narrow, with much of the bridge's great width blocked off with heat dissipating geometry to funnel heat away from the two-lane, 20ft wide road that crossers must brave. This crossing is not easy, however, as despite a these great and technologically impressive measures taken the 150ft journey is still an ordeal as throats are parched, lungs, mouths, and noses scorched and bit by terrible fumes, and exhausting, disorienting heat assailing the crosser from before they even set foot upon the structure. Special masks are required to avoid being poisoned by the fumes or contracting the dreaded Ash Lung, available for purchase from the small trading towns on either bank of these great bridges.

In the valley between these two bridges, and controlling them and their fares, is the Elven city of Dome. Its exorbitant fares to cross have made it wealthy, but driven many to seek alternatives that seem to come and go with each passing year. Still none have managed to fully replace them, however.

Well the other way is to use modern engineering, user. But then you're not playing in a medieval fantasy anymore.

Make the lava deep or far away that way the heat is less intense due to distance otherwise your gonna need to rely upon magic.

Build a wide, thick stone bridge with pipes that carry cold water through it to keep the bridge's road cool.

You should also note that you can come very close to lava without being horrifically burnt although it will be uncomfortable. Maybe the non-magical people could wear heat-resistant coats while crossing the bridge?

That sounds suitable.

Although note that heat is very different from fire, and there are quite a few cost-effective flammable materials that make good insulation.

You can also have magical super-ice that, while it still melts, lasts far longer than normal and creates an aura of cold that fights off the heat while crossing the bridge. Think Crystal Chronicles. Creation and transportation of these chunks of super-ice could be monopolized in the same way for economic control.

As for water pipes, the frames that support the gondolas could have water flowing through them, and metal connectors supporting these pipes all the way down to the bridge act as heat conductors from metal to water.