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Neponset River Estuary Assessment Report
Lynn Ficarra
Introduction
The Neponset River Estuary is a small, tidally dominated urban estuary located in the
Dorchester Bay region of the Boston Harbor, Massachusetts (Gardner, Chen, and Berry 2005).
The Neponset River is the second largest river flowing into the Boston Harbor, draining a 340
km2 watershed (Zhu and Olsen 2009, Gardner, Chen, and Berry 2005). While the watershed
encompasses 14 towns and cities, the estuary itself is located primarily in Boston and Quincy
(Cooke et al. 2003). The estuary is separated from the river by the Walter Baker dam and its
mouth meets Dorchester Bay 7 km from the dam (Figure 1, Breault 2011).
While organizations such as the Neponset River Watershed Association (nepRWA) have
extensively studied the freshwater components of the watershed, little is known about the
Neponset River Estuary. To assess the restoration needs of the estuary and to propose a
management plan, factors influencing the health of the estuary must be evaluated. The general
characteristics of the estuary (size, volume, tidal oscillations, etc.), dominant habitat types,
residence time, and nutrient limitation status are important factors that structure the estuarine
system. Understanding these features will help provide the tools to manage this delicate
ecosystem.
General Characteristics
The Neponset River Estuary is approximately 7 km long and ranges from 25-250 m wide
at low tide and up to 1 km at high tide (Gardner, Chen, and Berry 2005). The depth of the main
channel near the mouth is about 4 m at low tide, but may be less than 2 m in some locations
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(Gardner, Chen, and Berry 2005). The tidal range is approximately 3 m, thus a large portion of
the water in the estuary is exchanged at each tide (Zhu and Olsen 2009). The surface area is 2.17
km2 at high tide and 0.88 km2 at low tide (Gardner, Chen, and Berry 2005). River flow through
the Walter Baker dam and into the estuary ranges from 0.6 to 3.1 m3/s (Breault 2002). This
information is useful in categorizing the estuary based on depth, tidal versus river flow, and
salinity stratification.
As a shallow, tidally dominated estuary, the Neponset estuary is either a partially or wellmixed estuary with the latter being most likely. Though the estuary type is not explicitly stated in
the literature, NOAA Ocean Service Education characterizes a well-mixed estuary as shallow
with low river flow, moderate to strong tidal currents, and no vertical stratification. The
Neponset estuary is shallow, river water inflow to the estuary is slowed by the Baker dam, and
tidal currents are strong. These provide support for the well-mixed theory; however, the vertical
stratification of the estuary is not well known. To determine the extent to which the estuary is
vertically stratified, salinity measurements could be taken at varying depths at either end of the
estuary and in the center. This may be complicated by seasonal variation in stratification, thus
year-round sampling will provide a clearer picture of salinity stratification in the estuary.
To the best of my researching capabilities, the total volume of water in the estuary is
unknown. Due to variations in tidal flow, river flow, watershed inputs, estuary depth, estuary
width, etc., mathematically estimating the volume of water within the estuary at any given time
is very complicated and beyond the scope of my abilities. However, it would be worthwhile to
create a model estimating the volume of water in the estuary through time. This would involve
several measurements including mapping the bottom topography, plotting the dimensions of the
estuary, and charting tidal and river flow variations. Knowing the volume of water is useful for
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estimating residence time and quantifying water exchange during each tidal cycle. This has
implications for nutrient loading and the potential for phytoplankton blooms.
According to Gardner, Chen, and Berry (2005), ice covers the estuary during the winter.
This is an example of a seasonal variation in the condition of the estuary. Ice cover can alter
many characteristics of an estuary such as salinity, temperature, wave action, dissolved oxygen
concentrations, etc. Seasonal variability in the estuary would be a useful factor to study because
management strategies may need to be adjusted seasonally with changing ecosystem dynamics.
Habitats
The Neponset River Estuary contains the largest remaining salt marsh habitat in Boston
Harbor (Berault 2002). In addition to salt marshes, the Neponset estuary contains shallow mud
flats and large stretches of intertidal zone with approximately 29% of the intertidal surface area
being non-vegetated (Figure 2, Gardner, Chen, and Berry 2005). Historically, the salt marshes
were dominated by native Smooth cordgrass (Spartina alterniflora) and Saltmeadow cordgrass
(Spartina patens), but the invasive Common reed (Phragmites australis) has taken over and
caused a decline in Spartina (NepRWA 2006). In the past, wastes from dredging the river river
were dumped into salt marshes causing the loss of native organisms and an opportunity for
invasive species to move in (Tiner et al. 1998).
In 1991, the Neponset River Estuary had 311.64 acres of wetlands comprised of 25.92
acres of non-vegetated wetland, 265.67 acres of salt marsh, and 20.05 acres of common reed
marsh (Tiner et al. 1998). According to the 2004 nepRWA Assessment Report, all of the
remaining salt marshes within the estuary have been severely degraded. It would be very helpful
to actually measure the amount of wetlands that currently exist in the estuary to determine how
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much it has changed since the 1990s. This will indicate the scope of the damage that has
occurred to wetland ecosystems over the past few decades.
Other than the presence of mud in the mudflats, the sediment characteristics of the
Neponset estuary are not well known. While there have been some sediment sampling used to
measure nutrients and bacteria, there is no literature describing the characteristics of the bottom
substrate. Sediment size and composition as well as levels of turbidity and sedimentation are
important habitat descriptors. Sediment particle size and composition may affect which
organisms can live within or closely associated with the substrate. Certain particle sizes are more
or less likely to be disturbed by turbidity. Sedimentation is important for water clarity which
relates to primary production. I theorize that sedimentation rates are low near the river because
the dam reduces the flow of sediments into the estuary. Likewise, I theorize that turbidity and
sedimentation are increased near Dorchester Bay because of tidal currents. Sediments in shallow
areas are more likely to be affected by wind-driven surface waves which mix sediment into
suspension and reduce water clarity. Sediment studies are important for understanding water
clarity issues.
The Neponset River Estuary is home to a diverse group of organisms. In addition to the
previously mentioned salt marsh plants, the estuary is an important habitat for several
anadromous fish species. Historically, rainbow smelt, herring, and American shad used the
estuary and Neponset River for fish runs which supplied fisheries (nepRWA 2004).
Unfortunately, river dams have greatly shortened the distance the fish can travel. This along with
pollution has caused the fish populations to decline (Massachusetts Government 1998). The
Neponset estuary is also, an important nursery habitat for young commercial species such as
winter flounder, herring, clams, bay scallops, conch, dogfish sharks, spiny dogfish sharks, skates
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(nepRWA 2006). As a result, predators such as bluefish and striped bass use the estuary for
hunting (nepRWA 2006). Estuaries provide many ecosystem services which is why management
and restoration is crucial to marine and freshwater ecosystems.
Residence Time
According to the NOAA Office of Coast Survey, residence time is the average amount of
time a substance remains within a physical system. In the case of an estuary, residence time (or
flushing time) refers to the amount of time a drop of water remains within an estuary before
being flushed out into the ocean. Flushing time of an estuary is important because it influences
the concentrations of nutrients and pollutants in the estuary which directly affects the health of
estuarine organisms (Aikman and Lanerolle 2004). In an estuary with a short residence time,
nutrients and pollutants within the water column will be quickly removed from the system.
Longer residence times allow for increased concentrations of chemicals as water is retained in
the system for longer periods of time.
Geographic features contribute to the residence time of an estuary. The Neponset River
Estuary is shallow and tidally dominated (tidal exchange may involve the entire estuary) which
results in a short time scale for mixing and transport within the estuary (Gardner, Chen, and
Berry 2005). Gardner, Chen, and Berry (2005) describe the estuary as a microestuary which they
define as having a total length of less than 10 km and having a short residence time of one day or
less. The literature does not, however, provide a more precise calculation of residence time for
the Neponset estuary.
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Nutrient Limitation
A water quality assessment of one sampling station within the Neponset estuary yielded
high levels of bacteria, nitrogen, and orthophosphate (Cooke et al. 2003). While this study was
lacking in number of sampling sites and was vague about what exactly constituted a “high” level,
it is useful in suggesting that nutrient loading and bacteria levels may be a concern. To
understand the water quality of the estuary, further studies must be conducted. According to the
Massachusetts Government (1998), the estuary is included in a 1998 list of impaired waters due
to organic enrichment, low dissolved oxygen, pathogens, and turbidity. This report is outdated,
though, and should be updated.
According to the nepRWA 2004 Assessment Report, the estuary is generally limited by
nitrogen. It is difficult to quantify reasonable nutrient levels, though, because the Massachusetts
Surface Water Quality Standards do not include specific thresholds for nutrient concentrations
(nepRWA 2004). Instead, nutrient level standards are described via vague qualitative guidelines.
This makes it nearly impossible to regulate nutrients in the estuary. Potential sources of nitrogen
include atmospheric deposition, sewage, livestock waste, and fertilizer runoff (Castro et al.
2003). Nutrient inputs will not be controlled unless government can set up very specific
regulations.
Conclusions
There are many gaps in our knowledge about the Neponset River Estuary. These gaps
include the volume of water in the estuary, sediment characteristics, salinity stratification, and
residence time estimates. The literature tends to focus primarily on the rivers in the Neponset
watershed rather than the estuary. Much of the information needed to properly assess the status
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of the estuary and propose a management plan is unknown. There is a miniscule amount of
current literature discussing the estuary and scientific research is especially scarce. In general,
available information about the Neponset estuary is outdated and new studies should be initiated.
Some projects have been created to improve conditions in the Neponset River Estuary.
The Neponset River Watershed Association and other organizations are researching the potential
environmental effects of removing some of the river dams. Eliminating dams will help
anadromous fish populations recover (Breault 2002). Another project that nepRWA is working
on is the restoration of salt marshes in the estuary (nepRWA 2006). Though these projects are a
great start, the estuarine ecosystem is complex and requires workers to continue thinking outside
of the box in terms of management strategies. Two strategies that warrant investigation is
whether eelgrass and/or oysters may benefit the ecosystem. In addition, how much eelgrass or
how many oysters will it take to clean up the estuary?
Figures
Figure 1: From Breault 2011. The Neponset River
drains into the Neponset River Estuary which
meets the Dorchester Bay in the Boston Harbor.
Figure 2: From Gardner, Chen,
and Berry 2005. The medium gray
represents the extensive intertidal
region surrounding the estuary.
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References
Aikman F. and L.W.J. Lanerolle. 2004. Report on the National Ocean Service Workshop on
Residence/Flushing Times in Bays and Estuaries. Office of Coast Survey, NOAA. 25 p.
Breault R.F. 2002. U.S. Geological Survey Quality Assurance Project Plan for Bottom Sediment
Thickness and Quality, Lower Neponset River, Massachusetts, with Analysis of Potential
Contaminant Sources. U.S. Geological Survey, Water Resources Division MassachusettsRhode Island District. 89 p.
Breault R.F. 2011. Concentrations, Loads, and Sources of Polychlorinated Biphenyls, Neponset
River and Neponset River Estuary, Eastern Massachusetts. U.S. Geological Survey
Scientific Investigations Report 2011-5004, 143 p.
Castro M.S., C.T. Driscoll, T.E. Jordan, W.G. Reay, W.R. Boynton. (2003). Sources of Nitrogen
to Estuaries in the United States. Estuaries. Vol. 26(3):803-814.
Cooke I., L. Larson, C. Pawlowski, W. Roemer, S. Woods. 2003. Boston Harbor Watersheds
Water Quality & Hydrologic Investigations. Executive Office of Environmental Affairs,
Massachusetts Department of Environmental Protection, Bureau of Resource Protection.
Project No. 2002-01/MWI. 377 p.
Gardner G.B., R.F. Chen, and A. Berry. 2005. High-resolution measurements of chromophoric
dissolved organic matter (CDOM) in the Neponset River Estuary, Boston Harbor, MA.
Marine Chemistry. Vol. 96:137-154.
Massachusetts Government. 1998. Neponset River Subwatershed-River and Estuary Segment
Assessments. Boston Harbor Watershed 1999 Water Quality Assessment Report. p. 87188.
NepRWA. 2006. Neponset Salt Marsh Restoration. The Neponset River Watershed Association.
http://www.neponset.org/Restoration-SaltMarsh.htm
NOAA. Last Revised 2011. Estuaries. NOAA Ocean Service Education.
http://oceanservice.noaa.gov/education/pd/estuaries/welcome.html
NOAA. Determination of Residence Time in Estuaries. NOAA Office of Coast Survey.
http://www.nauticalcharts.noaa.gov/csdl/residencetime.html
Tiner R.W., D.B. Foulis, C. Nichols, S. Schaller, D. Petersen, K. Andersen, and J. Swords. 1998.
Wetland Status and Recent Trends for the Neponset Watershed, Massachusetts (19771991). U.S. Fish and Wildlife Service National Wetlands Inventory Project Northeast
Region. 32 p.
Zhu J. and C.R. Olsen. 2009. Beryllium-7 atmospheric deposition and sediment inventories in
the Neponset River estuary, Massachusetts, USA. Journal of Environmental
Radioactivity. Vol. 100:192-197.