Penobscot River, a major river of Maine. It begins in northeastern Maine at the junction of the East Branch and West Branch Penobscot rivers and follows a curving 100-mile (160-km) course southward to Penobscot Bay, an inlet of the Atlantic Ocean. The combined length of the West Branch Penobscot and the Penobscot is some 350 miles (560 km), the longest river system in the state. Chief tributaries are the Mattawamkeag and Piscataquis rivers. The river is a source of power for pulp and paper mills and is used for recreational purposes. Oceangoing vessels dock at Bangor, the river's principal port.