The current boundary line is determined by the Treaty of Ghent made December 24th 1814. The international boundary is set right down the middle of the river between lakes Erie and Ontario. With this division, the U.S. ended up with most of the islands, however Canada has Navy Island.
In 1909 the Boundary Waters Treaty of 1909 came into being to resolve any future boundary disputes. During the shaping of Terrapin Point, the U.S. Army Corp of Engineers expanded the land area outward from Terrapin Point since the water was flowing to the center and the flanks were dry. This put all of Horseshoe Falls in Canada.
Other older treaties having to do with the boundaries were the Paris Peace Treaty made September 3rd, 1783 and the Jay Treaty made November 19th, 1794. The boundaries have remained the same since before the War of 1812.