RE:leaving an isolated part of a circle called an "oxbow" lake. There are many of them in the US.

well, i was taught that meanders and oxbow lakes are charactericts of old rivers.. so you find a lot of them on the Mississippi, and but not so much on the Ohio. (the Ohio river is now part of the Mississippi drainage, but there is lots of geological evidence, that it wasn't always, and that at some time in the past it flowed into great lakes.)

meanders and oxbow lakes are less common in New England, since many of the rivers and river beds moved during last ice age, so many of the rivers are newer-- speaking on a geological time scale.