Some rivers form the border between two territories and affect units crossing in either direction. An example is the Dnieper between Kiev and Eastern Ukraine. It doesn’t matter which side of the black border line the river runs - it’s the actual border and affects attacks from either direction.
Then there are rivers that do not form the border and only affect attacks from outside the territory, coming in. An example is the Amazon in Amazonas. If someone was attacking from Venezuela into Amazonas, they would face the river penalty. But if someone was attacking from Amazonas to Venezuela they would not suffer the river penalty.
My question is about when the distance from the river to the border becomes so large that we no longer declare the river to be “forming the border.” For example, the border of Rhodesia and Angola. Is the river denoted the Zambesi? The Lungwebungu? Is that river supposed to lie on the border? I don’t think so, but just wanted to confirm. At first, it is on the border, but then it falls away and the distance becomes too great. I would think if someone attacks from Angola to Rhodesia, they face the river penalty, but not vice versa.
What if the distance from the river to the border becomes a little smaller than that? Take the Lena river on the border of Northern Yakutia and Eastern Yakutia. I would judge the distance from the river to the border to be too far to “form the border,” but perhaps I’m misunderstanding this? Again, I’d suggest if someone attacks from Northern Yakutia to Eastern Yakutia, they would face the river penalty, but not vice versa.
But how about the Lena river on the borders of Angara, Irkutsk, and Buryatia? Now, the river is getting closer to the borders. Lake Baikal clearly separates Angara from Buryatia. It seems like the distance of the Lena from the border between Angara and Buryatia, then continuing between Irkutsk and Buryatia, should be the breakpoint for how distant a river can be from the border and still be able to define the river as “forming the border” so the penalty applies both ways. But as the Great Lebowski pointed out, that’s just my opinion, man.