Saint John, river, 418 mi (673 km) long, rising in N Maine and flowing NE to New Brunswick, Canada, then SE below Edmundston, past St. Leonard, Grand Falls, Woodstock, and Fredericton to the Bay of Fundy at St. John. It forms part of the border between Maine and New Brunswick. Its chief tributaries are the Aroostook and Tobique rivers. At Grand Falls the river drops 75 ft (23 m) in a great cataract. At its mouth, within the city of St. John, are the Reversing Falls Rapids, caused by the strong tides of the Bay of Fundy, which force the river to reverse its flow at high tide. The river was visited (1604) by the French explorers Samuel de Champlain and Sieur de Monts. In the 17th and 18th cent. it was an important route for French, Native American, and English traders, and several trading posts were established on its banks. It later became a major lumber transportation route. There are major hydroelectric power plants at Grand Falls, Beechwood, and Mactaquac. The river is navigable to Fredericton. The valley of the St. John is fertile, and potatoes are raised there.