I think most rivers look like that given enough time and erosion. I don't remember the explanation but I half remember reading that just about any curvy river has carved its riverbed into that shape over very long periods.
water moves faster along the outer bends, giving it an increased capacity for carrying sediments. this results in erosion happening faster near the outer curves. on the other hand, water is slower near the inner bends, forcing the sediments carried by the river to deposit there. this interaction makes the rivers more bendy. an interesting result of this phenomenon is the formation of oxbow lakes (those c-shaped bodies of water separated from the main river)
People saying "ya it's called natural borders yank" as if they don't come from countries where borders are decided instead by whoever was fucking over whichever disenfranchised minority group at any given time.
Also I actually think borders should be set at watershed boundaries instead of rivers since a lot of cities end up sprawling onto both sides of a river divide. The continental US should have 8 states,
Longhouse at the North Eastern watersheds,
Sherman along the southeastern, Tennessee Valley, and lower Mississippi watersheds
Ohiagan along the Ohio, Great Lakes, Upper Mississippi, and Sourris Red watersheds
Texas along the Rio-Grande and Texas Gulf watersheds
Buffalo along the Missouri and Arkansas watersheds
Sierra along the Colorado and Great Basin watersheds
I invite you to look up the town of Baarle-Hertog/Baarle-Nassau and surrounding area. I would love to describe the area as "on the Dutch/Belgian border", except there isn't really much "on" going on there. Seriously, have a look and tell me I'm wrong.
Linked because I haven't yet learnt to put screen shots in comments.
I live opposite to the slime. My border is nicer, but we live under constant threat of meeting people from the shitty part of Virginia, so it's not much better.