Mhh, thinking about it, I think there are some merits in using topography even in relatively flat regions compared to the river borders, although not always.
If someone needs a base map for the US, this should be good:
For the Appalachians, while I wouldn't follow watersheds exactly I wouldn't try to incorporate all the elevated areas but try to put the eastern borders at the closest peakes rather than the foothills if they don't contain any importnat territory. I'm not going to do a full map, because it's kinda frustrating that I don't have a mixed topography watershed map, but I think the suggestion made here are solid overall, maybe factoring in the topography the map above shows could make some borders better, although there is still this issue of connectivity vs. similar geography which are 2 relatively opposing philosophies, I'm partial to the first, although I can see the problem it could create if it's prioritized too much.
If someone needs a base map for the US, this should be good:
For the Appalachians, while I wouldn't follow watersheds exactly I wouldn't try to incorporate all the elevated areas but try to put the eastern borders at the closest peakes rather than the foothills if they don't contain any importnat territory. I'm not going to do a full map, because it's kinda frustrating that I don't have a mixed topography watershed map, but I think the suggestion made here are solid overall, maybe factoring in the topography the map above shows could make some borders better, although there is still this issue of connectivity vs. similar geography which are 2 relatively opposing philosophies, I'm partial to the first, although I can see the problem it could create if it's prioritized too much.