How can I explain, in a believable way of course, that country borders closely resemble drainage divides? They don’t always match mountain ridges, which are natural borders – as are the rivers themselves in our world.

In particular, with a point of divergence (POD) ±50 years around our calendar epoch (Julius Caesar, Jesus Christ), how could this have happened in Europe so that about a millenium later borders looked somewhat like the red lines in the map of drainage basins shown below? In some cases, especially for rivers draining into the Baltic and Black Seas, I would additionally use the gray lines. The white areas would belong to either neighboring area or be independent.

European Watersheds

Note that the exact position of lines depends on how you define your water bodies (seas, oceans, rivers), so it could look slightly different in Denmark/Sweden, Greece, Italy and Scotland.

Just for the record, I’m planning to establish the following countries with somewhat Latin names for some degree of (partially anachronistic) familiarity:

  • Atlantic Ocean
    • Norvegia (Norway)
    • Islandia (Iceland)
    • Hibernia (Ireland)
    • Britannia (Wales + Scotland + Cornwall): Severn
    • Gallia (France): Seine (Belgica) + Loire (Lugdunensis) + Garonne (Aquitania)
    • Lusitania (Portugal): Douro (Galicia) + Tejo (Castilia) + Guadiana + Guadalquivir (Baetica)
  • Baltic Sea
    • Suecia (Sweden + Finland)
    • Baltica (Latvia + Estonia): Neva + Narva + Daugava
    • Polonia (Poland + Lithuania): Oder + Vistula + Neman
  • North Sea
    • Anglia (England) Ouse + Trent + Cam + Thames
    • Germania: Elbe (Saxonia), Weser, Ems (Frisia), Rhein (Alemannia), Maas (Belgica)
  • Mediterranean Sea
    • Catalania/Tarraconensis: Ebro, Baleares
    • (Massilia): Rhône
    • Italia: Tiber, Corsica, Sardinia, Sicily
    • Graecia: Vardar (Macedonia), Mariza (Thracia)
  • Adriatic Sea
    • Venetia: Po
  • Black Sea
    • Moesia/Istria (Bavaria + Austria + Hungary + Serbia + Romania etc.): Danube
    • Dacia: Dniester + Bug
    • Ucraina/Sarmatia: Dnieper
    • Caucasia/Iberia: Don
  • Caspian Sea
    • Russia: Volga
  • Polar Sea
    • Lappia: Dvina, Pechora
  • $\begingroup$ This is probably more suited for chat, but ... that is a beautiful karte. Did you make it yourself? $\endgroup$
    – Mikey
    Commented Feb 8, 2016 at 22:19
  • $\begingroup$ @Mikey Sorry, I forgot to link to the source. Fixed. It’s from Wikipedia (but only available as PNG, not SVG, sadly). $\endgroup$
    – Crissov
    Commented Feb 9, 2016 at 8:19

5 Answers 5


Trade and military logistics

Before the invention of the steam machine (and even for a time after that), the most economical way to transport goods were river ships and barges. So, trade was more natural with other powers down and upriver that with other regions.

So, one option would be making your society focused in trade more than in agriculture (note that it means a sizeable surplus so there is enough to trade). Instead of being the possession of hereditary nobility (whose intermarriages and politics would complicate the distribution of loyalties) make the town and regions ruled by a plutocracy interested in promoting trade; to that effect they will try to have good relationships with neighbours along the river.

It would help if there were noticeable differences between the products offered by shore provinces (v.g. sea fish) and those offered by the inner provinces to estimulate trade.

Another option would include a military artifact/s that is decisive in battle but too massive to be moved long distance, except by barges (think of a mix of siege cannon to storm castles and machine gun to win pitched battles, or a war animal who cannot forage and must have its forage brought through the river). Once a faction controls the river to move such a weapon as it wishes, the rest of the regions bordering the river must surrender to it.

In both cases it would be nice making open sea navigation dangerous enough so that countries do not find too profitable trade between different watersheds (maybe profitable enough for luxury goods that have a high margin, but not for bulk goods). This would make a coastal only empire (who makes no attempt to trade/invade the interior of the watershed) less likely.


Lots and lots and lots and lots and lots and lots of mountains.

Prior to the invention of flight, mountains were often more than a little bit annoying to cross. Passes make traversing a mountain range easier, but the journey can be treacherous if the roads aren't good. In a society in a more medieval era, crossing the mountains is tough. Clearly not impossible by any means, but tough.

If you have trouble crossing a mountain range, it doesn't make sense to have more land on the other side be part of the same country. If the society has not yet figured out how to use telegraphs or telephones, messages need to be sent by land. This makes governing hard.

You can use mountains to separate watersheds. States will be reluctant to have regions separated by mountains be under the same government. If they are governed together, I predict that secession would be easier. It's tough to get an army over the mountains, and the existing army on the other side might not be big enough to suppress a rebellion.

The feasibility of this does depend on there being a low level of technology in the region.


Take a look at Brazil.

The original boundaries of Brazil included the entire watershed of the Amazon river. It was a very nice and logical way to claim land from the sea: you claim a river and all the land that feeds it.

It's not too unbelievable that humans would accept the idea for claiming any land would be to claim all the water that flows from it to the sea. From a resource protection standpoint this makes a lot of sense.


All watersheds are not alike. In our world, major rivers often form the core of a state or district and are not borders. However, smaller rivers form good borders and are not useful as a core of any substantial region.

And if you have river travel, you have sea travel, which tends to unify lands more than your watershed state does. Italy, for example, is usually split across the boot rather than right down the middle. Sometimes a statelet forms a border in the Apennines, but soon travel across and down the boot changes the situation to the more normal situation.

Your states have huge lateral expanses rather than cross from one watershed to another near the source. There's no good reason for this situation to persist long - if you can solve the problem with a 2000 mile wide state, you can solve travelling 50 miles across the top of a watershed much easier.



The point of divergence would be right in the (alternate) bible: Jesus goes to John the Baptist, but there is no baptism because the river has been fouled by some heathens upstream. Instead, Jesus holds a sermon about river water being the gift of life and purity from God.

After that, the expanding Christian culture makes sure to always protect their rivers right up to the springs, leading to your country borders.


Alternatively, someone came up with the idea of poisoning whole rivers as a form of chemical or biological warfare. Given the state of science back then, it would probably have been an accidental discovery.

For example, an earthquake exposes some kind of weird-colored soil and every fish in the stream it fell into died for many days travel downstream. A devious general/rebel/villain then carts off barrels of the goo and dumps it in the springs of his rival kingdom's main river.

After that, all water sources will be of military importance and no country will leave another in control of their water sources.


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