river that rises as an outlet of the Chiputneticook Lakes, Maine, U.S., and flows south-southeast into Passamaquoddy Bay, an inlet of the Bay of Fundy (Fundy, Bay of); its entire 75-mile (121-km) length forms the international boundary between Maine and New Brunswick, Can. Navigation upriver is possible for only 10 miles (16 km) because of power dams at Calais-Milltown. The river, once important economically for lumbering, is now used for recreation. Given its name by Jacques Cartier (Cartier, Jacques), who saw it on Sept. 14, 1535 (St. Croix 【Holy Cross】 Day), it was visited in 1604 by other French explorers, Pierre du Guast, sieur (lord) de Monts, and Samuel de Champlain (Champlain, Samuel de), who founded an ill-fated colony on St. Croix Island at the river's mouth.

tributary of the Mississippi River, rising in Douglas county, Wis., U.S., in the northwestern part of the state. It flows 164 miles (264 km) southwestward into the Mississippi River at Prescott, Wis., about 20 miles (32 km) south of St. Paul, Minn. For 130 miles (210 km) upstream from its mouth, it forms the boundary between Wisconsin and Minnesota. At Taylors Falls, Minn., and at St. Croix Falls, Wis., the river cuts through Interstate Park; parts of the river are designated a national scenic waterway. A hydroelectric plant at St. Croix Falls furnishes power for St. Paul and Minneapolis.