I know what a watershed is, we’re talking about borders between states.
Draw a squiggly line that follows the river. You get the left side, I get the right side. The river moves as rivers do. After 100 years we do a survey and find I have a big chunk of property that used to be my side of the river but the river moved towards me and now that’s on your side of the river. I’ve lost some land due to some being on the other side of the river and more due to the land I used to have being a river now.
Is the boundary between us the water and we put up with the uncertain nature of our property, or do we honour the line drawn 100 years ago even though the river isn’t anywhere near there now?