My wife was an attorney in Las Vegas 15-ish years ago. As the town grew out into the desert, land that hadn’t been surveyed in a hundred years suddenly became valuable for housing tracts.
Two major brand-name builders (who hated each others’ guts) bought adjacent parcels of about a square mile apiece and began grading and leveling for construction of their latest marvels of soulless ticky-tack.
That’s when they discovered their parcels had an overlapping area about 100 yards wide and the length of their common border, nearly a mile. On paper, the plots were adjacent. On the ground, measured with modern tools from the definitional markers, they overlapped.
Oops.
Wife made a good living for a couple of years off of solving that one. Funny how neither party could see that splitting the difference and getting on with construction would have made them both a lot more money than idling all that capital for years to fight over who got 1 more row of houses. Heck, by trimming the lot sizes a smidgen they could both have fit just many houses in there as per their original plans. But noooo.
Human nature; it’s what lawyers depend on for job security.
As to rivers shifting course versus legal land boundaries …
There’s a legal distinction between gradual change, called “accretion”, and sudden change (ie after a flood or earthquake) called “avulsion”. I’m not the expert here, but IIRC, under accretion the border moves with the river, but under avulsion it remains where it was.
So around here, as the Mississippi slowly moves left or right from year to year, Illinios shrinks and Missouri grows in spots, or vice versa. But after a flood and the river settles into a new course, there may be political “islands” formed on the newly other side of the river.
One such political island is in north Omaha NE, where a chunk of Iowa was marooned on the west side of the current river course after an old near-loop in the river course broke through. http://www.mapquest.com/maps/map.adp?searchtype=address&country=US&addtohistory=&searchtab=home&address=&city=carter+lake+&state=ia&zipcode= and click zoom level 7.
The same logic applies to the landowners along the riverbank. Their owned parcel is constantly shrinking or growing as the river gradually moves. But if it jumps the banks, they may become a political island in themselves.