Boundary Peak is an unnamed mountain on the Canada-United States border between Maine and Quebec, with one half in Oxford County, Maine, and the other in Le Granit Regional County Municipality, Quebec. Its importance is geographic: it is the highest point on the international border east of the Rocky Mountains, and specifically along three thousand one hundred ninety miles of border from Passamquoddy Bay to one hundred eleven degrees west in Liberty County, Montana. The mountain also forms a major drainage divide. Runoff from its south and east sides reaches the Kennebago River and ultimately the Gulf of Maine, while water from its north and west sides flows through Quebec waterways to the Gulf of Saint Lawrence.