The problem with rivers as frontiers is one that's particularly prevalent in terms of the number of people who still make maps with the St. Lawrence as an international border despite it making no sense at all.
I think there's scope in a Venn diagram of people who started out making maps this way, and people who started out making maps when the only available online base maps were so low resolution/otherwise poor quality that defaulting to rivers became the obvious choice for making a 'different to OTL border that looks realistic'.
I've done it, but as you say it makes makes very little sense once you spend more than a few moments considering the role of rivers in human history.
Not that it wouldn't be enjoyable to see a satirical "Lower Nile as an international border" map.
Thing is, you can make it work for some rivers, but the St. Lawrence in particular has the issue that it's navigable by ocean going vessels for so far down the length that controlling Montreal and Quebec (both on the north side) means you have complete sway to exert control over the south bank, and controlling one or the other lends itself to a lateral division rather than following the river.
Egypt's similar but less so in that at least you could use Alexandria as a major base to get a border on the delta, but Cairo does just dominate both sides of the river there.
There just isn't the strategic depth of settlement that you could have with, to take an example, an Elbe boundary.
Agreed. I wonder if there's an analogous "mountain pass" type settlement that would work to exert the same kind of strategic control over both sides of what, on paper, looks to be an obvious dividing line.
Another problem is skmetimes the river itself. Rivers are an economic resource, especially for water. It's why California and Arizona are in interminable conflicts over water rights, since much of their mutual border is along the Colorado River.The problem with rivers as frontiers is one that's particularly prevalent in terms of the number of people who still make maps with the St. Lawrence as an international border despite it making no sense at all.