Where do the Kennebec Atlantic salmon hide?
Prior to the removal of the Edwards Dam in 1999, it was common to see large
numbers of Atlantic salmon and trout congregating at the mouth of Bond Brook
in Augusta during the hottest part of each summer (mid July - mid August).
Because it is fed by springs and groundwater, Bond Brook maintains a much
cooler temperature than the Kennebec River during mid-summer. Prior to
the removal of the Edwards Dam in 1999, a deep pool at the mouth of Bond
Brook, just below the stone arch bridge on Water St., provided an area large
enough for up to 100 salmon and trout to lie in cool water during the heat
of the summer.
Salmon and trout would not be seen in this pool until water temperatures
in the Kennebec River exceeded 70 F, usually in mid-July. By mid-August,
water temperatures in the Kennebec River drop back below 70 F and the salmon
and trout congregating at the mouth of Bond Brook would disperse back into
the Kennebec River.
Breaching of the Edwards Dam, one fourth mile upstream of Bond Brook, commenced
on July 1, 1999. Removal of the dam structure continued through the summer
and fall. Fish were not able to migrate upstream past the dam until a large
(200 foot wide) breach was completed on August 12, 1999. This breach restored
the 15 mile reach of the Kennebec River impounded by the Edwards Dam to
its natural elevation, depth and flow.
Summer 2000 marked the first hot weather season in over a century when Atlantic
salmon were able to inhabit the 20 mile reach of the Kennebec River between
the former Edwards Dam in Augusta and the Lockwood Dam at Ticonic Falls
in Waterville, Maine.
Repeated observations (often daily) have been made at the mouth of Bond
Brook during the summers of 2000, 2001, 2002, 2003 and 2004. Throughout
the past five summers since the Edwards Dam was removed, not a single salmon
or trout has been observed at the mouth of the Bond Brook during hot weather
periods. Prior to the removal of the Edwards Dam, up to 100 Atlantic salmon
and trout could be observed seeking the cool water at the mouth of the brook
during hot weather periods.
What might explain this sudden change?
We begin with a process of elimination.
Bond Brook has not changed. The brook and its mouth were not affected by
removal of the Edwards Dam, one fourth mile up river. Bond Brook maintains
the same flow and temperature as it did prior to removal of the Edwards
Dam.
Numbers of salmon and trout in the river do not appear to have changed.
Salmon are observed leaping in the river from Augusta to Waterville and
spawning at various locations above Augusta. Trout in the river are frequently
caught by anglers.
Climatic conditions have not changed. The summers of 2000, 2001, 2002, 2003
and 2004 have been similar in temperature, if not hotter and dryer, than
summers in previous years.
What has changed?
Since 2000, the Edwards Dam is no longer a migration barrier to salmon and
trout.
Removal of the Edwards Dam has restored the natural channel, depth and flow
of the 15 miles of the Kennebec River formerly impounded by the dam; and
restored free access to the entire river to Ticonic Falls in Waterville,
Maine.
Removal of the Edwards Dam has restored access to more than a dozen first
and second order tributaries of the Kennebec River between Augusta and Waterville;
and portions of three larger tributaries, Seven Mile Stream, Messalonskee
Stream and the Sebasticook River.
In 2000, 2001, 2002, 2003 and 2004 these tributaries have been visited repeatedly
during mid-summer to determine their potential and actual use as thermal
refugia for salmon and trout in the Kennebec River. The following observations
have been made.
All of the first and second order tributaries are either dry or at extreme
low flows in mid summer. Those not dry have water depths of one to six inches
with few if any accessible pools of sufficient size to hold salmon or trout.
Of those not dry in mid-summer, measurements with a pocket thermometer show
no appreciable difference in water temperature between the tributaries and
the Kennebec River. No trout or salmon have been observed in or at the mouths
of these tributaries in mid-summer seeking thermal refugia.
Three larger tributaries of the Kennebec River above the Edwards dam site
are of sufficient size to accommodate Atlantic salmon and trout seeking
thermal refugia during mid summer. These are Seven Mile Stream, Messalonskee
Stream and the Sebasticook River.
Seven Mile Stream is similar to Bond Brook in size, with channel widths
of 15-25 feet and pools of sufficient depth to hold an adult Atlantic salmon
or trout. The first dam on Seven Mile Stream is at the outlet of Webber
Pond, three miles upstream from the Kennebec River. Seven Mile Stream receives
most of its flow from a series of natural, warmwater ponds east of the Kennebec.
Mid-summer water temperatures in Seven Mile Stream are equal to or higher
than water temperatures in the Kennebec River.
Messalonskee Stream is much larger than Seven Mile Stream, with a drainage
of 200 square miles, channel widths of 50-80 feet and numerous pools with
depths of 6 to 12 feet. The source of Messalonskee Stream is a group of
large, natural lakes and ponds, known as the Belgrade Lakes, located 10-15
miles west of the Kennebec River. All but the lowermost mile of Messalonskee
Stream is impounded by a series of hydro-electric dams which are impassable
to fish. Mid-summer water temperatures in Messalonskee Stream are higher
than in the Kennebec River.
The Sebasticook River is the largest tributary of the Kennebec, with a drainage
area of nearly 1,000 square miles. An impassable dam is located on the Sebasticook
approx. 1,400 feet above its confluence with the Kennebec River. The Sebasticook
is fed by many large and small natural lakes. The lowermost eight miles
of the river is impounded by two hydro-electric dams. Mid-summer water temperatures
in the accessible portion of the Sebasticook are consistently higher than
in the Kennebec River.
The review above indicates that none of the tributaries to the Kennebec
River in the 20 mile reach from Augusta to Waterville, Maine appear to offer
thermal refugia for Atlantic salmon and trout. These tributaries are either
too small and intermittent to hold Atlantic salmon or have mid-summer water
temperatures warmer than the Kennebec River itself.
Despite the removal of the Edwards Dam, the only accessible Kennebec River
tributary with sufficient mid-summer flow, depth and cool water to provide
thermal refugia for Atlantic salmon and trout is Bond Brook in Augusta.
Yet, since the removal of the Edwards Dam, not a single Atlantic salmon
or trout has been observed using the thermal refugia at the mouth of Bond
Brook, even during hottest days of the summer.
What has caused this change?
During mid-summer "hot spells" water temperatures in the Kennebec
River can reach 75 F. These are temperatures known to cause Atlantic salmon
and other salmonids to seek thermal refugia.
This creates a conundrum. None of the Kennebec tributaries above Bond Brook
in Augusta appear to offer any mid-summer thermal refugia for salmon and
trout. At the same time, since removal of the Edwards Dam, salmon and trout
have completely abandoned their decades-long habit of congregating in large
numbers in the thermal refugia at the mouth of Bond Brook. Prior to removal
of the dam, up to 100 Atlantic salmon and trout sought thermal refugia in
the mouth of Bond Brook, even though the mouth is in a highly visible, urban
area frequented by salmon poachers.
By a process of elimination, this information suggests that since removal
of the Edwards Dam, Atlantic salmon and trout have located mid-summer thermal
refugia in the Kennebec River itself.
In some ways, the 20 mile reach of the Kennebec River from Waterville to
Augusta confounds standard assumptions about large New England rivers. It
is normally assumed that first, second and third order tributaries to the
lower reach of a large river (5,000 square miles) should be cooler during
mid-summer than the large river. Observations and temperature data for the
lower Kennebec since removal of the Edwards Dam indicate the main-stem of
the Kennebec is often cooler during mid-summer than the small tributaries
entering into it.
Several observations made since the removal of the Edwards Dam may shed
light on this.
The first observation is that removal of the Edwards Dam and restoration
of the river's natural channel has revealed numerous spring seeps along
the river's banks from Augusta to Waterville. Many of these seeps, located
slightly above or below the river's edge, maintain temperatures of 50-60
F throughout the summer. The surficial geology of this portion of the Kennebec
River suggests much of the precipitation falling in the watershed adjacent
to the river enters the river as groundwater rather than surface flow. Much
of the 20 mile reach of the Kennebec from Waterville to Augusta consists
of thick deposits of well sorted, permeable sand and gravel interspersed
with layers of marine clay. A large and pronounced glacial esker forms the
west bank of the river from Sidney to Augusta (approx. 10 river miles) that
is intensively mined for sand and gravel. These geological features, coupled
with the observation that most first and second order tributaries to the
Kennebec in this area become nearly dry during summer, suggests groundwater
inputs to this reach of the river may be significant.
The second observation is that mid-summer water temperatures in the restored
river channel vary significantly as one moves across the channel perpendicular
to the river's flow. During mid-summer, near shore, shallow areas (less
than 1 foot in depth) are noticeably warmer than the "center channel"
which is deeper and has much higher flow velocities. Similar to a deep lake
which stratifies vertically during mid-summer, the Kennebec appears to be
stratify horizontally, with the center of the channel noticeably cooler
than near-shore areas in mid-summer. This makes recording mid-summer water
temperatures in the Kennebec problematic, since a temperature reading taken
near the river's bank would be significantly different than a reading taken
in the river's center channel. That temperature-sensitive fish such as
Atlantic salmon and trout would take advantage of these differences in water
temperature across the river channel appears self-evident.
This discussion does not attempt to provide definitive answers to the questions
it raises. Instead, it documents a sudden and dramatic change in the behavior
of Atlantic salmon and trout in the Kennebec River coincident with the removal
of the Edwards Dam; and examines factors which may have caused this change.
Since removal of the Edwards Dam, Atlantic salmon and trout of the Kennebec
River have abandoned their well known thermal refugia at the mouth of Bond
Brook. Because we know Atlantic salmon and trout are still present in the
Kennebec River, we can surmise that with removal of the Edwards Dam, they
have located thermal refugia elsewhere as good or better than Bond Brook.
However, none of the accessible tributaries of the Kennebec River above
the Edwards Dam site appear to offer any thermal refugia. By the process
of elimination, the only location for thermal refugia must be the Kennebec
River itself.
Qualitative observations of the lower Kennebec River in the four years since
removal of the Edwards Dam provide several potential explanations. Observation
of numerous springs and seeps along the river suggest groundwater inputs
to the river may be significant. The post-glacial history and surficial
geology of the river reach are consistent with these observations. The
observation of a mid-summer, horizontal water temperature gradient across
the river channel suggests a segregation of river flow into warmer and cooler
'threads,' providing salmonids the opportunity to selectively inhabit those
portions of the river channel best suited to their physiological requirements.
It is also possible many other factors are at play as well.
There is somewhere an explanation for why Kennebec Atlantic salmon no longer
enter the cool water of Bond Brook anymore during the heat of the summer.
I believe the Kennebec salmon have it.
Douglas Watts
Kennebec River
Augusta, Maine
February, 2005.
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