Diceman – after a too-long search of the Web, the only thing I’ve seen it refered to as is “11 square miles of Fulton County, Kentucky.”
I knew we discussed this before - I stumbled across a message board full of border freaks, and it appears that the bit of Kentucky we were discussing here is called (rather mundanely) the “Kentucky Bend”, or “New Madrid Bend”.
That phrase yields plenty of results:
There’s not enough discussion of Western state border oddities, so I’ll throw in my own: Point Roberts. When the US and UK (with the help of the Kaiser) settled the US-Canada boundary, it was set to run straight out across country till it hit the middle of Strait of Georgia. Problem was, there’s this little tongue of land hanging off Canada that just barely crosses the 49th parallel. It’s completely cut off from the rest of the US by land and is quite small. Most land is now owned by Canadians as vacation property. It is too small to support a school so the kids have to take a long bus ride each morning to the US “mainland”, crossing the border twice each way.
And indeed, Idaho is the leftover bits that other states didn’t want. The orginal Oregon territory included OR, WA, ID and bits of MT and WY (including a good size chunk of what is now Yellowstone NP). That’s not counting southern BC, of course. WY got squared away, MT got a nice little piece, as prev. noted. OR was carved out first, leaving a “7” shaped figure. The Columbia and Snake rivers formed most of the north and west boundaries. The non-river parts are extensions of lines sort of paralleling the rivers before they turn off. WA was next carved out and it’s eastern boundary was set about the same longitude as OR’s. That left a really odd little Idaho. The north and south (most populous) parts are nearly cut off from each other by mountain ranges, practically a Michigan. The territory should have been divided into East/West of the Cascades. (Centrally located Ontario would have made a perfect capitol.)
Oh, and there are some islands in the Columbia river that are disputed between OR and WA. One of them, the stunning Beacon Rock, neither state really wants.
I believe you are mistaken. I have sailed those waters and do not remember any time where the Potomac is split. I have in front of me the national Geographic map titled Close up USA #10, Maryland, Virginia, Delaware and West Virginia published in Jan 1978, scale 1:886,000 and it very clearly shows the border on the Virginia side. Maybe there is some part in the very upper part of the river where the width is not discernible? Can you find a some specific confirmation? Otherwise I have to say I believe you are mistaken.
The state line was defined as the low water mark. In the 1930s gambling was legal in MD but not in VA. A number of gambling dens were built on pilings in MD waters on the VA side of the river. Legally they had to be separate from the shore so there was a break of just a couple of inches on the walkway and the border was marked “Entering the State of Maryland”. They had MD police which arrived by boat. I saw an interesting page about this but cannot locate it now. It had photos and long story.
At any rate, I am quite sure the border is the low water mark on the VA side of the river.
Small, teeny-tiny correction.
The Columbia River does indeed form most of Oregon’s northern border, but the Snake River forms most of the eastern border, not western.
The Pacific Ocean does the job of squaring away Oregon’s western border. 
Same for Maryland. Pennsylvania, Delaware, and Virginia didn’t want it. There weren’t any big important cities Baltimore hadn’t been formed yet and Annapolis wasn’t very impressive.
Actually, Edward the Head was correct in the first place. I am looking at the ADC Northern Virginia map book, 43rd ed. (2001). It clearly shows places just below Washington where the Md.-Va. border does not hug the Virginia bank but is out in the water.
Just below Alexandria south of Jones Point (where the Beltway’s Woodrow Wilson Bridge is), down to near Fort Hunt, there’s a stretch of river where the border cuts through the water instead of hugging the bank, putting some of the river in Virginia.
There’s an inlet of the Potomac at Mount Vernon, next to the mouth of Little Hunting Creek, which is in Virginia.
On either side of Fort Belvoir, there are two inlets of the Potomac (estuary of Dogue Creek and Gunston Cove) in Virginia where the border cuts across the water.
From Hallowing Point to the edge of Mason Neck NWR is another inlet of the Potomac within Virginia waters, and then from High Point across Occoquan Bay and on down, the border stays in the water, touching the Virginia shore only at Freestone Point until it gets to Cockpit Point. That’s as far as this map goes, though my Rand McNally shows the border including several more Potomac inlets in Virginia farther downstream.