[jmilloy]

I think borders based on watersheds make so much more sense than those on rivers, both of which are common.

[NegativeLatency]

Either seem better than arbitrary lines

[emmelaich]

Harder to find though.

[londons_explore]

The big bit of a river is easy to find, sure.

But up near the source where it splits into thousands of small tributaries which move and change a lot? That's a recipe for boundary disputes.

Whereas a watershed rarely changes far, even on geological timescales.