When the boundary line between the United States and Canada was determined in 1819, based on the Treaty of Ghent, the northeastern end of the Horseshoe Falls was in New York, United States, flowing around the Terrapin Rocks, which were once connected to Goat Island by a series of bridges. In 1955, the area between the rocks and Goat Island was filled in, creating Terrapin Point.[5] In the early 1980s the United States Army Corps of Engineers filled in more land and built diversion dams and retaining walls to force the water away from Terrapin Point. Altogether, 400 ft (120 m) of the Horseshoe Falls was eliminated. Due to erosion, the Falls will continue to move in relation to the boundary line in the future, possibly altering territorial boundaries between the two countries.
The official national maps for both Canada and the United States indicate that a smaller portion of the Horseshoe Falls currently is located within the United States.[6][7]
^ ab"Niagara Falls". World Waterfalls Database. Retrieved January 28, 2017.
^"The international boundary line may pass through the Horseshoe Falls just beyond Terrapin Tower Point, so that the United States are not entitled to as large a portion as Canada of the Horseshoe Falls, which is admittedly not all Canadian." Statement of Mr. J Boardman Scovell. Preservation of Niagara Falls: hearings on the subject of H.R. 26688, Sixty-first Congress, second session, relating to the control and regulation of the waters of Niagara River and the preservation of Niagara Falls, held before the Committee on Rivers and Harbors of the House of Representatives. U.S. G.P.O., 1911