Apparently King George declared the boundary in 1764 amidst the NH/NY controversy over who controlled Vermont, and they wouldn't let it into the Union unless the Vermont legislature affirmed the border, which it did in 1782, but Vermont still wasn't let in because New York until 1790. And no one really paid attention until New Hampshire tried to tax a mill that was partially in the river in 1917, and there was two decades of litigation, and a SCOTUS case in 1933 affirming the border, and also instructing the Attorneys General of each state to meet at the border every 7 years to make sure they still understood, and they
still do this (and several states once had
laws that every town must perambulate its borders every n years).