Caribou Mountain is a border peak in far western Maine whose summit ridge extends into Quebec, with its high point in Franklin County and the ridge’s southwest end in Le Granit Regional County Municipality. It stands on the Canada–United States boundary where the line follows the height of land between major watersheds. That divide makes the mountain significant: water from its Maine slopes flows by branches of the Moose River into the Kennebec River and the Gulf of Maine, while runoff on the Quebec side travels through the Rivière aux Araignées, Lac aux Araignées, Lac Mégantic, and the Chaudière to the Saint Lawrence River and Gulf of Saint Lawrence.