Over
the last 30 years, pollution in the
Neponset
River
has been cleaned up, the buildings along its shores have been
converted to residential and commercial rather than industrial
use and its banks have been opened up and improved for
recreation. All of these changes have dramatically increased
the river’s importance as an aesthetically pleasing backdrop
that people can enjoy as they go about their daily business.
Without question, any dam removal project changes the
appearance of a river, though the change is often not as
dramatic as one might at first imagine. Aesthetic concerns are
also the most subjective issue that needs to be evaluated when
considering whether to keep or remove a dam. The Watershed
Association believes that dam removal presents a unique
opportunity to re-engineer the
Neponset
River
with the specific goal of enhancing the river’s aesthetics.
The
first and most challenging task in evaluating river aesthetics
post-dam removal, is to accurately visualize the change before
it occurs.
What would a free-flowing
Neponset
River
look like?
The
short answer to this question is that a free-flowing
Neponset
River
will look like, more or less, the parts of the Neponset
River
that are free flowing right now. Take the Watershed
Association’s visual tour of the
Neponset
River
(to come) to see for yourself.
When a dam is removed, the water levels behind the dam will
drop vertically, and in most cases the river will be narrower
than the previous mill pond. However, removing the dam does
not change the amount of water that is constantly coming down
the river every day, so that although the depth and width of
the former impoundment will change, the river itself will not
dry up. The amount by which the depth and width of the river
will change depends on the shape of the riverbed, both across
its width and along its length.
A mill pond typically presents a slow moving, lake-like
appearance. Furthermore, most mill ponds create at least the
appearance of a more or less constant depth and width
throughout the seasons. By contrast, a free flowing stream
presents a more dynamic appearance, as the river meanders back
and forth and works its way over or around obstacles. The
appearance of a free flowing river typically varies
considerably across the four seasons as the discharge of the
river varies from spring snowmelt to summer dry periods.
Another critical factor in determining the appearance of a
free flowing river is the slope of the riverbed. The steeper
the riverbed, the faster the water moves. Free flowing rivers
with a low slope tend to be slower moving, wider, deeper,
meandering and marshy with soft muddy bottoms. A steeper
riverbed translates into faster moving water. When a given
quantity of water speeds up, it generally gets narrower and
shallower. The faster moving water also tends to scour away
fine muddy deposits, leaving behind a stream bed of coarse
sand, gravel or boulders and producing the proverbial
“babbling brook” or even a white water rapid.
Natural processes tend to produce rivers that change character
at regular intervals as you travel from upstream to
downstream. Rather than being all slow moving or all rapids,
rivers tend to adopt a natural variation with a slow moving
“pool” followed by a fast moving “riffle,” and then a
smoothly flowing “run.” This pattern is so common that it
is referred to as the “riffle, run, pool” pattern. This
variation creates visual as well as auditory interest. It also
creates a diverse array of micro-habitats that helps account
for the tremendous diversity of plant and animal species that
can be found in and along a free flowing river. In a dam
removal context, it is possible to partially control the
formation of riffles, runs and pools by intentionally
configuring the riverbed.
Because dam removal makes the river narrower, it also creates
new areas that will be colonized by riverside plants and
wetlands that were drowned out when the pond was created. The
character of these “new riverbanks” will vary depending on
the steepness of the riverbank. In some areas where the
riverbanks are very steep, the width of the river will hardly
change at all and there will be no new vegetation. Other
areas, which are more gently sloping will tend to be marshy or
grassy if they are inundated much of the year, shrubby if they
get flooded at least a few times per year, and wooded in areas
that are generally inundated less than every other year.
The “new” riverbanks created following dam removal will
re-vegetate rapidly, even without any active replanting
effort. The pattern of vegetation can be controlled by
adjusting the shape of the stream channel or by periodic
pruning in drier areas. The newly exposed areas will not smell
unless there is pollution getting into the stream (in which
case a pond would smell too).
The best way to really understand how these visual changes
might play out is to take a tour of the Neponset
River
as it exists right now, and see with fresh eyes what the river
really looks like in areas where it is and is not dammed. In
lieu of an actual field trip, the watershed Association has
assembled a “virtual field trip slide show.” (soon to
come)
In addition, the Riverways Program has commissioned a limited
number of “photo simulations” (soon to be posted) that
attempt to approximate the appearance of the river post-dam
removal.
Who can see the river now?
Given
that aesthetics are “all in the eye of the beholder,” it
is also important to understand the perspectives from which
various users currently gain “visual access” to the river.
Under existing conditions, there is virtually no visual access
to the T&H Dam and very little visual access to the
T&H Impoundment (i.e. the T&H Mill Pond). Visual and
physical access to the T&H Dam is completely blocked by
the former paper mill on the
Boston
side of the river and by abandoned industrial land on the
Milton
side. The impoundment is largely fenced off, and can be seen
only by a few abutting homeowners and from a few locations
such as near the
Martini
Shell
Park
off
Truman Parkway
, and where bridges go across the river. Because of this
limited visual access, to date no one has indicated to the
Watershed Association that they would object to the removal of
the T&H Dam because of aesthetic concerns.
There is significantly more visual access to the Baker Dam and
its impoundment. During the winter and early spring, the Baker
Dam can be seen by looking upstream from
Adams Street
. However the dam is not visible from
Adams Street
during the summer because of the vegetation. The Baker Dam and
Impoundment can be seen from the Neponset Greenway Trail along
the south side of the pond and the public can approach the dam
itself from the south as well. In addition some units of the
existing Baker Square Condominiums have a view of the Baker
Impoundment and a few have views of the Baker Dam.
June
2008
