Two different stories
Years ago, I read the the book "How the States Got their Shape". If my memory is correct, the reason was that there was a political feud between two politicans. One got "exiled" to a remote part of the terrority to be a judge. Since he had better connections in Washington, he got the boundary moved to get back at his opponent.
The second story is from the website of the Idaho State Historical Society.
Idaho looked like a rectangle when it became a territory. How did it get its present odd shape?
Originally, Idaho consisted of all of Idaho, Montana, and most of Wyoming, an unwieldy land mass. Much of this area had been part of Washington Territory, and Idaho was created because western Washington politicians wanted to discard vast tracks of land to the east which were gaining population due to gold rushes. This action would leave a more compact Washington Territory—and help to assure that Olympia would remain as its capital. When Idaho’s first territorial legislature met in Lewiston in 1863, one delegate from present-day Montana, unwilling to venture across the Bitterroots in winter, instead journeyed overland to California and then by boat up the Pacific and the Columbia and Snake rivers to the proceedings. Not surprisingly, those first legislators unanimously petitioned Congress to reduce the size of the territory to make it more governable, not to mention easier to get around. In 1864, Montana broke away, and in 1868, Wyoming left, leaving Idaho with its jagged panhandle in the north and a nearly straight-edged boundary with Wyoming in the east. Check out this
fact sheet for more information on Idaho's territorial maps.
It is my understanding that the mountains in that part of the country and very challenging to cross especially in winter.
Has any one else read something different?
Stubear1012