What Happens To A Border If The River Changes Course?

The River Lys (Leie, in Flemish) has formed the border between Belgium and France for nearly 200 years. The only problem is: rivers move. So what happens to the border when the river takes a new course? What does it look like? And with hundreds of river borders around the world, how do different countries deal with it? I went to northern France / southwestern Belgium to investigate...

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French coat of arms: https://commons.wikimedia.org/wiki/Fi...