St. John River, the principal river of the province of New Brunswick, Canada. It begins in northwestern Maine and for about 70 miles (113 km) forms part of the United States-Canadian boundary. It then flows southeastward through New Brunswick and empties into the Bay of Fundy at Saint John. The river is 418 miles (673 km) long. Tributaries include the Aroostook, Tobique, Oromocto, Canaan, and Kennebecasis rivers.
The St. John and its tributaries provide much of New Brunswick's hydroelectric power. The river is navigable to Fredericton, the provincial capital. Of the numerous cataracts on the St. John, 75-foot (23-m) Grand Falls is the most impressive. Near the river's mouth are the famous Reversing Falls, a rapids through which river water flows downstream at low tide and seawater flows upstream at high tide.
