Not to criticize, but I'll say it again since I've said it before in maps that have 'riverine' borders.
In general, it's very rare nations have borders following rivers. There are, of course, some places a river border is used (like the Danube between Romania and Bulgaria, or the Rio Grande), but in general something like a crest of mountains is much more often used.
The reason is economics. Generally, river valleys are among the most productive land for agriculture. Pre-railroad, they were also by far the best places for trade access. Though there's sometimes times you want a sister city across the border, in general putting such valuble resources so close to a potential enemy is a dangerous gambit.
For example, on your map, Texas or the United American Kingdom could stop all trade up and down the Mississippi with minimal effort. On the other hand, if North America has been largely at peace for generations, borders like this don't matter much, as people and goods travel freely across the borders anyway.
It would be much more likely for there to be riverine boders in the U.S. than Europe though, given long-standing ethnicities didn't exist which crossed the borders. After all, the Rio-Grande was used. On the other hand, I just don't think you'd see a Mississsippi split up like that unless it fell apart based upon internal borders from a much larger American nation, since settlers would either come up the Mississippi, or from the east.
Just my $0.02. Nice map though
