Watersheds and rivers are often key determinants of pre-industrial borders, if only because they served (variously) as obstacles (wide, fast rivers) and facilitators (narrow / slow) to navigation. Until the development of steam-driven railroads and internal-combustion-driven vehicles, if you wanted to move any considerable quantity of goods or people, your best option was a ship over either a (reasonably) calm sea, or inland rivers, lakes, and canals. Ice ran a close second, and ice roads remain vital transport routes in Siberia and Alaska. Where run over rivers and lakes, those tend to be (near) level terrain, and frictional losses are minimised.
China built over 1,000 km of canals 1,000 to 2,000 years ago. Inland travel within England and Great Britain was revolutionised by the development of canals. The US opened up its interior with the Erie Canal, on which a single mule could haul an 80-foot barge laden with 40 tonnes of goods or people. Not only was the travel efficient (in terms of motive power and drovers) but it was comfortable. A four-foot-deep ditch (the Erie Canal) was immune to the rutting and mud of a road, and though the speed of travel was low, in a matter of a few days people could travel in great comfort across New York from the Hudson to Lake Erie and from there to points inland via the Great Lakes, Chicago, and Mississippi / Missouri / Ohio / Red, and other, rivers.
I'd realised the political boundaries aspect looking at a map of European watersheds created by Robert Szucs:
It's not that national borders are specifically identified as watersheds, but in large part you can patch together current state boundaries out of watersheds and rivers, or find internal divisions (e.g., UK counties, German states, French provinces) out of watersheds.
In regions in which borders were more arbitrarily assigned, as with the inland US, Middle East (o hai Sykes-Picot), and Africa, the artificiality of imposed borders is highly evident, though regional affiliations become clearer.
South American countries are, as you note, more closely aligned with watersheds. It's also more evident why the north-eastern region of South America is so politically divided: it's largely composed of non-contiguous watersheds which have become their own polities.
I could have been clearer that both descriptions were meant to be or statements.
Fast rivers or wide rivers are challenging, the former as they're inherently treacherous, the latter as they're difficult to bridge.
Narrow rivers can be bridged. Slow rivers can be traversed by boats, ships, barges, etc.
Yes, wide rivers tend to be slow, and narrow rivers tend to be fast, all else constant, but all else is rarely constant such that there are narrow slow rivers and wide fast / treacherous ones.
China built over 1,000 km of canals 1,000 to 2,000 years ago. Inland travel within England and Great Britain was revolutionised by the development of canals. The US opened up its interior with the Erie Canal, on which a single mule could haul an 80-foot barge laden with 40 tonnes of goods or people. Not only was the travel efficient (in terms of motive power and drovers) but it was comfortable. A four-foot-deep ditch (the Erie Canal) was immune to the rutting and mud of a road, and though the speed of travel was low, in a matter of a few days people could travel in great comfort across New York from the Hudson to Lake Erie and from there to points inland via the Great Lakes, Chicago, and Mississippi / Missouri / Ohio / Red, and other, rivers.
I'd realised the political boundaries aspect looking at a map of European watersheds created by Robert Szucs:
<https://www.seattlepi.com/local/science/article/Viral-maps-s...>
(Europe is in the carousel at the top, third image, isolated here: <https://s.hdnux.com/photos/54/21/22/11605137/3/960x0.webp>)
It's not that national borders are specifically identified as watersheds, but in large part you can patch together current state boundaries out of watersheds and rivers, or find internal divisions (e.g., UK counties, German states, French provinces) out of watersheds.
In regions in which borders were more arbitrarily assigned, as with the inland US, Middle East (o hai Sykes-Picot), and Africa, the artificiality of imposed borders is highly evident, though regional affiliations become clearer.
South American countries are, as you note, more closely aligned with watersheds. It's also more evident why the north-eastern region of South America is so politically divided: it's largely composed of non-contiguous watersheds which have become their own polities.
<https://s.hdnux.com/photos/54/21/22/11605136/4/960x0.webp>