It arises from four branches in several lakes in the central Maine, and flows generally east. After the uniting of the branches, 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, Maine.
The United States government maintains three river flow gages on the Penobscot river. The first is on the East Branch in Grindstone, Maine where the rivershed is 1,086 square miles. Flow here has ranged from 37,000 to 77 cubic feet per second. The second is in West Enfield, Maine where the rivershed is 6,671 square miles. Flow here has ranged from 153,000 to 1,630 cubic feet per second. The third is in Eddington, Maine 0.4 miles downstream from the Veazie Dam where the rivershed is 7,764 square miles.
The river is heavily polluted by the nearby paper mills, which is disrtupting the Penobscot Tribe's way of life. They can no longer safely eat the fish from the water or rely on the river otherwise because the dioxin levels in the water are so high. In fact, the cancer rate on Indian Island is twice what it is in the rest of the state of Maine.
However, an effort is being made to restore the river's habitat for sea-run fish, while still maintaining its very important energy production. http://www.penobscotriver.org describes this ongoing effort, involving the removal of two very historic dams (the Great Works and Veazie dams), and the construction of a bypass ladder for the Howland Dam, started in 2004 in more detail.
The first European known to have explored the river was the Portuguese Estaban Gomez in 1524, followed by the Frenchman Samuel de Champlain in 1605. A few years later French priests came among the Penobscot people as missionaries and converted them to Catholicism. The French settlement of Pentagouet, now Castine was founded at the point where the river becomes Penobscot Bay, and the Penobscot people made a permanent settlement at Indian Old Town, on an island above the head of navigation, around the Catholic mission. Throughout the 16th and half of the 17th centuries, these were likely the only permanent settlements on the river, although the Penobscots considered the entire river and bay their hunting ground and maintained other seasonal villages along its banks.
In 1669 the Mohawk tribe made raids from the west that were very destructive to the Penobscot people. The English settlers in Massachusetts also sent periodic raiding parties to the Penobscot in this period but were not strong enough to wrest the area from French control until the 1750s. In a treaty of 1752, however, Massachusetts laid claim to the entire Penobscot watershed, and in 1759 the Pownall Expedition, led by the Governor of Massachusetts, established Fort Pownall on Cape Jellison in what is now Stockton Springs. This signaled the beginning of English domination, and the incorporation of the Penobscot River valley into New England.
The first permanent English settler on the river was Joshua Treat(1726-1802) who was initially the armorer and translator at Ft. Pownall. His oldest son Joshua Treat Jr. built a log house and sawmill at Marsh Bay in what is now Frankfort, Maine, and other members of their extended family, joined by additional settlers from Massachusetts and New Hampshire, pushed ever further up-river, eventually restricting the Penobscot people to Indian Old Town, the present Penobscot Indian Reservation.
The river and bay were the site of battles in both the American Revolution and the War of 1812. In both cases the British navy won, and in 1814 they sacked the town of Bangor. To prevent this from happening a third time, and because the nearby boundary between the U.S. and British Canada was still contested into the 1840s, the Federal government began constructing a huge granite fort, Fort Knox opposite the town of Bucksport, Maine, near the mouth of the river, in 1844. The fort never fired a shot in anger, but remains one of the Penobscot's major man-made landmarks.
In the 19th century the river was converted into a conveyor belt for the transport of logs from the northern woods, to be sawn into lumber at mills around Old Town and Orono, and transported on ships from Bangor, at the head of tide. A secondary economic use made of the river late in the century was as a source of sawn ice for urban markets.
In the 20th century, lumbering was largely supplanted by paper-making, in the form of large wood pulp and paper-mills located all along the river from Millinocket and East Millinocket in the north, to South Brewer and Bucksport in the south. The development of cheap hydropower also attracted other types of light manufacturing, like textiles and shoes.
In the 21st century, with the sudden decline of the Maine paper industry, and the divestiture of its woodlands, the Penobscot watershed is becoming more and more associated with recreational use (fishing, hunting, boating, and tourism) and less with manufacture.