Estuarine / Marine Contact • Background • Current Research
STEERING COMMITTEE CONTACT:
BACKGROUND:
The Penobscot River Estuary
 The Penobscot River estuary extends approximately from Bangor to Searsport/Bucksport where it meets Penobscot Bay. During extreme high tides and low flows, the tide may reach as far as Eddington Bend. With a surface area of over 23,000 acres, the Penobscot River estuary is the largest estuary in Maine and part of one of the largest embayments on the East Coast. The Penobscot is also a significant freshwater inflow to the Gulf of Maine, discharging 10 billion gallons per day (Hasbrouck, 1995).
Haefner (1967) conducted the first baseline study of hydrographic conditions in the estuary, from Bangor to Searsport. The Maine Cooperative Fish and Wildlife Research Unit continued to sample through 1977 (Townsend, 1985). Temperature, salinity, and dissolved oxygen were measured at five stations between Bangor and Bucksport. The estuary is classified as partially mixed. River flow and tidal flow are balanced; salt and fresh water mix to some degree, although the lower layers of water remain saltier than the upper layers. Salinity is greatest at the mouth of the estuary and decreases upstream. The extent of salt water moves up and downstream tidally and seasonally, depending on freshwater flows. During high flows in April, fresh water can extend as far south as Winterport. As river discharge decreases and flood tides reach further up the estuary, salt water can be found in Hampden and Brewer. Salinity in the estuary is between 17 and 20 ppt; marine water has been observed in bottom layers of the estuary as far north as Bucksport (Haefner, 1967).
The Penobscot estuary is a classic drowned river valley: the prehistoric Penobscot began as glacial meltwater, meandering east and west before finding its current course. The river carved a valley through granite and metamorphic rock, a valley which was later inundated by the rising postglacial sea.
 Joe Kelley, a professor at the University of Maine, has said that "Rock is the distinguishing feature of Maine estuaries." That's because the Maine coast is characterized by bedrock that can be seen all along the shorelines of the Penobscot estuary, from the overhanging cliffs between Hampden and Winterport, to the walls of Fort Knox built of Mount Waldo granite, to the rocky islands and outposts of the bay. One does not see in the Penobscot the kind of extensive muddy flats and marshes characteristic of other estuaries. In the rocky-shored Penobscot Bay, sediment supply is limited. Marshes exist only as a fringe bordering a few very protected coves and streams, such as the Marsh River and Bagaduce River (Jacobson et al., 1987).
Estuary Facts:
Mean depth: 29 ft (8.8 m)
Max depth: 101 ft (30.8 m)
Surface area: 23,600 acres (95.5 km2)
Volume (at MLW): 30 billion ft3 (850 million m3)
Mean tide: 6.5 ft at Bangor (2.0 m)
The transition zone between freshwater and marine environments is spatially small, but plays a disproportionately large role in the ecosystem. The estuary is a rich feeding environment for birds and fish, and the Penobscot's eelgrass, salt marsh, and mudflats provide critical nursery habitat for countless species of fish larvae and adults. The estuary is the ecological bottleneck where migrating fish converge on their way upstream. The estuary is also a region of intense biochemical transformation of nutrients and contaminants (Casper, 2004).
Significant human features of this section of the river include the Bangor Dam/Waterworks, the cities of Bangor and Brewer, former Eastern Fine Paper mill, former Holtrachem plant, PERC incinerator, Pine Tree Landfill, Mount Waldo granite works, Fort Knox, the Waldo-Hancock Bridge, and Bucksport.
Maine DEP sampled salinity in the estuary as part of their 2002 modeling work. According to their data, from August 2001, there is negligible salinity from Bangor to South Brewer. The first quantifiable salinity is observed at the North Orrington Site. The average salinity observed were 5ppt at Orrington, 10 ppt at Bald Hill Cove, 16 ppt at Winterport, and 23 ppt at Fort Knox.
Figure 6: Penobscot Estuary Salinity
References:
Barlow, J. and J. Boucher. 1993. Penobscot Estuary Studies. Corning School of Ocean Studies, Maine Maritime Academy, Castine, ME.
Casper, A.F. 2004. From the river to the sea – initial responses of the lower food web of the Kennebec River to dam removal and some implications for the Penobscot River-Estuary. Penobscot River Science Forum, October 19-20, 2004, Orono, ME.
Haefner, P.A. 1967. Hydrography of the Penobscot River (Maine) Estuary. J. Fish. Res. Bd. Canada 24:1553-1571. PDF
Hasbrouck, S. 1995. Maine Rivers and Streams. Geographical Digest Series #6, University of Maine Water Resources Program, Orono, ME.
Hatch, R.W. 1971. Temperature, Dissolved Oxygen and Salinity Data for the Penobscot River Estuary, 1966-1970. Water Resources Center, University of Maine, Bangor, ME.
Jacobson, H.A., G.L. Jacobson, and J.T. Kelley. 1987. Distribution and abundance of tidal marshes along the coast of Maine. Estuaries 10:126-131. PDF
Townsend, D.W. 1985. Temperature, Salinity and Dissolved Oxygen Data for the Penobscot River Estuary, Maine, 1963-1977. Bigelow Laboratory for Ocean Sciences, Boothbay Harbor, ME.
Researchers:
- Joseph Kelley, University of Maine
- Lauren Sahl, Maine Maritime Academy
CURRENT RESEARCH
Salinity and temperature of the Penobscot estuary
Maine Maritime Academy partnered with NOAA-Fisheries to study salinity and temperature of the Penobscot estuary during spring fish migration. Marine science faculty and students at Maine Maritime use the Penobscot estuary as their laboratory for coursework. These studies have generated a diverse data set for the estuary, including information on suspended particles, particle composition and distribution, and the estuarine turbidity maximum. Ongoing work includes examining the hydrography of the river plume in Penobscot Bay and the measurement of currents in the northern bay. Contact Lauren Sahl, Maine Maritime Academy.
Remote Sensing
GoMOOS buoy F is located in Penobscot Bay near Rockland. As part of the GoMOOS program, and with another NASA program, the station monitors chlorophyll, solar radiation, ocean color, and particle scattering. Sensors in the Penobscot River monitor particulate and dissolved matter entering the bay, accompanied by chemical analyses of samples collected around the watershed. Contact Collin Roesler, Bigelow Laboratory.
Harbor and estuary assessments are available online from the Maine DEP.
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