How do countries usually handle rivers moving around? It looks like in the US for state borders they go with where the rivers used to be. Makes for some messy borders
At least between Finland and Sweden, in the Tornio/Torneå river, there are small adjustments to the border done on a regular basis, every 25 years. The previous border adjustment was in 2006 and the one before that in 1981.
The river changes a little bit as it erodes its banks, and the border is officially moved accordingly. This naturally results in minuscule changes in the land area of both countries, though in this case the changes cancel each other out in the long run. It's a friendly border.
not exactly river moving around, just a border demarcation style change after several decades, small island, 2 big states possessing thermonuclear weapons :
in the US whether the state border changes depends on the specific agreement between the states, which may in some cases go back to the colonial era (for virginia vs. maryland, along the potomac, it's the low tide mark on the virgina side except for tributaries - so if VA builds out into the river it doesn't become part of maryland). It gets quite difficult to amend these because both states have to agree, plus congress, for any revision of borders. It's usually just not worth it.
One well known dispute was between new jersey and new york, over ellis and liberty islands; the supreme court had to intevene on that one and I think currently both states have jurisdiction.
In the 19th century, New York was given jurisdiction over the original land and New Jersey had jurisdiction over the surrounding waters. As the island was expanded through land fill, the newly created land was treated as part of New York until the Supreme Court ruled that the original agreement between the states gave the filled in land to New Jersey which is most of the island today.
https://www.google.com/maps/place/Louisiana/@32.2167684,-91....