TriPolaractually has a point here. The last time a county – and state – boundary changed iwas a few seconds ago as you read this, in one of those places where the boundary between states A and B is the riverbank, with state A running to the water edge and state B owning the opposite shore and the riverbed. A square inch or two of dirt crumbled off the state-A riverbank, falling into the river – and into state B, and from Historicalfigure County into Indianname County, and shifting the boundary by that few square inches.
The latest changes by legal action, though, would seem to be what is found at Captain Amazing’s link to the Census Bureau list.
There have been several changes to the US-Mexico border along the Rio Grande. The most recent ones were in 1970. So presumably the corresponding Texas counties’ borders were changed.
As well as the cited “Peconic County” initiative on Long Island, there was also some stirrings to have Brookhaven Township in Suffolk County split off to form, you guessed it, Brookhaven County.
Does it really change when the riverbed moves? I don’t think so. Othersie then why do so many county and state boundaries remain unchanged on maps even though rivers have shifted? There are plenty of bits of Mississippi and even Illinois that are only accessable from the state across the river, due to river shifts. But the legal boundary didn’t change.
Kaskaskia, Illinois is a notable example of a town that’s now WEST of the Mississippi in Illinois.