Webster-Ashburton Treaty, 1842, an agreement that settled the long-disputed boundary between Maine and New Brunswick, Canada. The chief negotiators were Secretary of State Daniel Webster for the United States and Lord Ashburton for Great Britain. The treaty gave Canada five-twelfths of the disputed area. It adjusted the border near the source of the Connecticut River; at the north end of Lake Champlain; in the Detroit River; and in the Lake Superior region. The treaty also provided for unrestricted navigation of the St. John River by ships of both nations; mutual extradition of criminals; and cooperation between the two nations in suppressing the African slave trade.