Totally nothing to do with the Rio Grande-case, but I find it interesting seeing how borders are drawn when time goes on. Look at the original 13 states in the US. Lines are squiggly, and made with care after the terrain. Then, some time has passed, and the US started to grow eastwards. Then the borders were made quickly with rulers.

You see the same in Australia. NSW and Victoria is a bit squiggly for a while, but then the colonisers said "hand me the fucking ruler, cunt!"

Pretty much, yeah. A lot of property boundaries are defined in refererence to adjacent bodies of water. It makes sense too, otherwise you'll get weird edge cases where 3m^2 of land on the mexican side belongs to USA because the river drifted since 1850. What are you gonna do with that little plot? Swim over there and put a fence around it?

Yeah, pretty much.

One time there was also an island that appeared in the Rio Grande that some people claimed as another country with a flag and everything. The US military kicked them off of it.