Penobscot River Synthesis: River research physical setting

The Penobscot is the largest river in Maine and the second largest in New England (after the Connecticut ). The river drains an 8,592 square-mile (22,252 km 2 ) watershed, roughly one-quarter of the state's land area.

The mainstem of the river begins at the confluence of the East and West Branches at Medway and flows to Stockton Springs/Castine, where it opens up into Penobscot Bay .

The West Branch originates on the Maine-Quebec border near Sandy Bay Township and Penobscot Lake , in mountainous terrain 1,700-1,800 feet above sea level. The East Branch begins at East Branch Pond, northwest of Baxter State Park , in a lake-filled region 980 feet above sea level. The Telos Cut diverted flow from the Allagash to the East Branch basin.

Below Medway, the river receives the flows of the Piscataquis and Mattawamkeag rivers; smaller tributaries of the lower river include the Passadumkeag River , Sunkhaze Stream, Kenduskeag Stream, Souadabscook Stream, Cove Brook, Marsh River , and Orland River . In total, there are over 8,000 miles of rivers and streams in the Penobscot basin.

Piscataquis River joins Penobscot

The watershed contains over 1,600 lakes and ponds covering 266,000 acres; some of the larger ones include: Seboomook, Chesuncook, Grand Lake Matagamon, Katahdin, Debsconeag, Cold Stream, Sebec, Pushaw, Brewer, and Silver.

Sebec and Silver Lake

Geologic Origins of the Penobscot River

The Penobscot River we know today is not the same river seen by the first inhabitants of the region. While bedrock provides the foundation for the geology of the area, the landscape and the Penobscot River itself were shaped by the last glaciation. The weight of the vast Laurentide Ice Sheet depressed the Earth’s surface. As the ice receded, the ocean followed it inland, submerging the Penobscot Valley as far inland as Millinocket. Orono was under tens of meters of water. Icebergs calved from the edge of the retreating glacier and marine mammals expanded in range. A thick layer of bluish-gray silty clay settled out of the marine water, forming the poorly-drained Presumpscot Formation that now drapes over much of the Penobscot Valley.

As the burden of heavy ice slowly lifted, the land began to rise and the marine waters receded. During this period of "isostatic adjustment" around 12,000-11,000 years ago, the drainage system of the present-day Penobscot began to take shape. Initially, the Penobscot was a high-volume, high-velocity river, fed by glacial meltwater. During this period, sea level fell to 60 meters below present sea level, creating a high gradient stream capable of excavating a channel through the glacial overburden. In places, the river appears to have reoccupied older valleys that existed before the last glaciation. In other locations, the river flowed over a series of waterfalls and rapids, indicating a “youthful” channel. The clay-rich Presumpscot Formation, combined with poorly integrated drainage patterns, created water and marsh-filled basins. To the east and west of the river, the low relief areas in the central Penobscot Valley filled with lakes. The Pushaw Lake, Caribou and Whitten bogs were large expanses of open water (Almquist-Jacobson and Sanger, 1999). Between 10,000 and 9,000 years ago, a post-glacial forebulge rippled through the landscape, like a wrinkle in a carpet, and at first tilted the land to the north, interrupting the developing drainage patterns. As the bulge passed, southward drainage patterns reestablished. Over time, the lakes filled with cattail marshes and later bogs. The shift from open water to swamp and river changed the resources used by people and influenced how they moved through the landscape and where they lived.

This northward tilting in the early Holocene, prior to 9,000 years ago, also tipped Moosehead Lake into the Penobscot River drainage. Evidence of old shorelines and a northern abandoned outlet indicate that the lake drained through Northwest Carry into the West Branch of the Penobscot River (Balco et al., 1998). The Moosehead contribution may have increased the discharge of the West Branch by approximately 50% over present volumes. When the northward tilt was relaxed, the present day Moosehead outlet to the Kennebec was established. (Historical geologic evidence supports this: an old delta in the Penobscot stopped building with the loss of water and sediment as a result of the outlet switch, and a part of the Kennebec delta was reactivated with the addition of the Moosehead drainage.)

This early history shaped the Penobscot River of today. Much of the channel in the central Penobscot Valley is floored by Late Pleistocene/Early Holocene gravels (Dudley and Giffen, 2001) deposited by the high velocity, high magnitude river. Today, fine sediment deposits are found at tributary mouths and on the downstream ends of islands (Dudley and Giffen, 2001) as well as in the estuary and bay.

Almquist-Jacobson, H., and D. Sanger. 1999. Paleogeographic changes in wetland and upland environments in the Milford Drainage basin of central Maine, in relation to Holocene human settlement history. Current Northeast Paleoethnobotany 494: 177–190.

Balco, G., and D.F. Belknap. 1998. Glacioisostasy and lake-level change in Moosehead Lake, Maine. Quaternary Research 49:157-170.

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