High Quality Content by WIKIPEDIA articles! The Penobscot River is 350 mi (563 km) long, making it the second longest river in the U.S. state of Maine and the longest river entirely in Maine. Its drainage basin contains 8,610 square miles (22,300 km2). It arises from four branches in several lakes in the central Maine, and flows generally east. After the uniting of the West Branch with the East Branch at Medway, it flows south, past the city of Bangor, where it becomes navigable. It empties into the Atlantic Ocean in Penobscot Bay. It is home to the Penobscot people that live on Indian Island.