How Indiana Got Its Shape
Indiana was previously a part of the land that came back into British possession after the French and Indiana Wat (1754-1763). Then to American possession after the American Revolutionary War. Indiana first emerged as a territory and becoming a state in 1816.
Indiana’s Eastern Border: The boundary used to separate the eastern third of the territory, known as the Ohio Territory, from the Indiana Territory was that specified in the Northwest Ordinance of 1787: a line from the juncture of the Ohio and Great Miami rivers due north to the Canadian border. The southern half of this first American territorial division remains to this day as the eastern border of Indiana.
Indiana’s Southern Border: The land acquired in the French and Indian War comprised a kind of triangle, with the Mississippi River, the Ohio River, and the Great Lakes as its “sides.” A remnant of these boundaries can be found in Indiana’s southern border, which remains to this day, the Ohio River.
Indiana’s Western Border: The rush of settlers populating the Ohio Territory soon began populating the Indiana Territory as well. In 1805 Congress further divided the Indiana Territory, as it had anticipated in the Northwest Ordinance. That act stipulated that the Wabash River would form the lower half of Indiana’s western border. The Wabash was the ideal border because it divided Indiana and the newly created territory of Illinois just about evenly – that is, until it veers to the east. Consequently, the law called for a straight line to take over at Vincennes, heading due north to the state’s northern border. But the straight line segment of Indiana’s western border does not commence at Vincennes. It commenced more than 40 miles north at what appears to be nowhere in particular.
In fact, it is somewhere very particular. It is the northernmost point at which the Wabash River crosses the longitude of Vincennes. By commencing the straight line here, Congress preserved the same width that would have been achieved had the line commenced at Vincennes, but avoided creating isolated pockets where the Wabash would have crossed back and forth over the straight line as the river meandered east and west, resulting in islands of jurisdiction isolated by the river.
Indiana’s Northern Border: Initially, the northern border Of Indiana was that specified in the Northwest Ordinance, a straight east-west line that intersected the southern most point of Lake Michigan. But such a line left Indiana with no actual access to the Great Lakes, which had acquired new, enormous importance since the Ordinance because of the connection to the Erie Canal, and via the canal, access to the Atlantic Ocean. Congress therefore adjusted Indiana’s northern border, locating it 10 miles farther north where it remains to this day.
Source: How States Got Their Shapes by Mark Stein