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I've created a city at the joining point of two rivers, and I'm trying to determine the population ceiling. I'm just going for a general idea of relative size, and so far I've just been making towns and settlements get larger as I go downstream from the mountains to the ocean. This city I justified the existence of with the settlements established on its adjacent river's tributaries. There will be another, even larger city near where the river meets the ocean. However, I've begun thinking that it may be more logical to have this first city be the largest because it is central. This question is essentially: is upstream trade in my world viable enough to cause the more central location be more populated rather than the downstream location?

My world is a fairly standard dnd 5e feudal society, with a lot of leniency to make room for all the homebrew bits of magic I add in. There are no magical vehicles or beasts of burden that could be used as a fantasy equivalent to steam power.

Edit: I basically didn't realize that you could tow barges with animals and I thought that upstream trade was a lot harder than it actually is. Thanks so much for the answers guys, I'll have to start doing some more research into the economics of medieval cities.

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    $\begingroup$ Make your coastal area rather swampy or desert-like and the area around your city rather fertile, and nobody will wonder why it is the biggest city around. $\endgroup$ – Erik Mar 26 at 9:55
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Economic Geography

Transport economics is the driver. Geography is the context.

Sea / river transport is cheapest per tonne mile. Road transport is the most expensive. Some places you can only get to by water, some places only by road.

@AlexP and @Separatrix give great answers. It is quite common for a Metropolis to be serviced by an associated smaller port city.

If the hinerland geography is right (fertile uplands, valuable mineral resources, yadda yadda) traders in goods will look, not for the shortest route, but for the cheapest route to a viable market.

Then the market takes over.

I've got a pile of stuff at the river and an enterprising captain willing to buy it, do I keep ownership and ship it further, with all it's risks, or sell it now but make less profit? If I sell now I can go back and dig up / grow more stuff ...

Once you have trade you have traders, and voila, city.

It is entirely reasonable to have a large commercial city inland where the trade routes meet, 3 water routes and ? roads in your case. The more trade routes, the more varied the types of goods, the more opportunities for trade and value adding. Coal up one river, iron ore up the other, bingo - Sheffield. And why would I move either 1 km more than necessary before turning it into a higher value, lower mass (cheaper to transport) product?

Also remember that sustainable trade is always two way. After all, where do the picks that I'm using to dig this stuff up (or whatever) come from?

Lots of fat wealthy merchants waiting at the centre of least effort of the local transport network for the next money making opportunity.

Your major city should be at the nexus of the greatest number of trade routes.

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Rivers are essentially medieval railroads. They were the one and only method to move large amounts of cargo over land. Ships can either sail up the stream, if the river is slow enough, or barges can be towed up. Towing is very efficient compared to pulling a cart, due to the low resistance of water, and also there are no broken axles. Towing of course requires a tow road for the draft animals to walk on.

If your city either has a big enough river and/or tow roads, trading upstreams is perfectly viable

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    $\begingroup$ Also: rivers can be modified to help traders. Deepening and widening the river by dredging reduces flow rate and allows for larger boats, reinforcing the banks and towpaths allows for easier mooring and towing, and building weirs and lochs can reduce the gradient (and flow) on any given stretch if done right. Hell, if the river is even a bit tameable it can practically be turned into an unplanned canal. $\endgroup$ – Joe Bloggs Mar 26 at 9:32
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There's a technical term you're looking for: Navigable

(of an area of water) deep, wide, or safe enough for a boat to go through - source

How far upriver your waterway is navigable is the deciding factor in how far inland you can have your major city. This normally only runs as far as the tidal range of the river, however some slow flowing rivers such as the Seine or Danube are navigable far beyond the tidal range meaning cities can use the river for trading far inland of the sea.

One major advantage of being at the upper end of the tidal range of a river is that above that point the river will be fresh water, giving your city a water supply as well as trade access. Closer to the sea they may have limited fresh water, fundamentally limiting the size of the city you can have.

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The city I'm from, "Ulm" which is the first city on the River Danube where the River is navigable. Ulm was a pretty large and rich city, because it was perfect for trading. The Danube was easy to cross to until the point the River "Iller" joint. And after this, people were able to ship goods downstream. Ulm was famous for their "Ulmer Schachteln" (Ulms Boxes) which were very simple ships with no drive. They were used to transport goods fabricated in Ulm downwards into other cities down the river. When they reached their destination, the goods and the boats (the wood) were sold, and the merchants would travel with their salesmoney upstream by carriage.
This leads to lots of production and manufacturing in and around the city of Ulm, and where's work, there'll be people. So yes, it's pretty viable to have a large(r) city upstream.

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As others already said ships could sail upstream or could be towed. So upstream traffic works.

When it comes to the size of the city there are some other points you should keep in mind:

  • How is it with taxes? A city and a good fitting spot could tax everything coming from up the stream and thus be rich.
  • What is it with the rest of the logistics? Rivers are key to transportation. But there are cities in a day or twos reach of the river that depend on caravans to get goods from the river-trading. Said city could be a key point to switch from river to caravan to get to other cities a bit off.
  • For going up the stream you probably need other ships than going down. Something more fit to being towed and resist the stream. Those need to be build and the city with the best builders for ships probably has an advantage. And work for those working towards building ships and thus a reason for them to live in the city.

those things could reason into why a city up the stream is bigger then some more down.

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  • Paris is a very large city in the middle of France, somewhere on the river Seine. What's the name of the city at the mouth of the Seine? It's Le Havre. Paris has 2 million inhabitants, or 12 million in the entire metropolitan area. Le Havre has 200,000. Paris was by far the largest city in France in the last millennium and a half.

  • Cairo is a very large city on the Nile. It has 20 million inhabitants. At the mouth of the Nile is Alexandria; it has 5 million inhabitants. Note that in the antiquity, in Hellenistic and Roman times, Alexandria was by far the largest city in Egypt, but this changed in the Middle Ages.

  • Budapest and Vienna are large cities on the Danube. At the mouth of the Danube is Sulina, in Romania; it's a small town.

  • In America, the mighty Mississippi flows through St. Louis, Memphis, Baton Rouge and New Orleans; they are all large cities.

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    $\begingroup$ The Seine is navigable to Burgundy, it's a good river. $\endgroup$ – Separatrix Mar 26 at 9:49

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