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On Saturday, Oct. 15, I woke to a phone call from a
concerned citizen that believed the beaver damn between the upper swamp
and the lower swamp downstream from Lake Nipmuc was the cause of the
flooding on the lake. I decided to take a walk through the woods to see
for myself what was happening.
First, let me explain the hydrology of Lake Nipmuc and the surrounding
region. The lake and 3 nearby swamps are a system. There is a small
swamp just West of Kathy Larson's house that captures water from the
hills to the South of Lake Nipmuc and feeds it into the Lake through a
small stream that flows under Kinsley Lane.
Nipmuc's outlet is at the South West corner and flows through 2 36 inch
culverts
into a stream which overflows into the large upper swamp. At
the South West corner of the upper swamp it outlets through a gorge into
the larger lower swamp. In the gorge there is a beaver damn.
There is a second gorge that descends from Millville Road to the upper
swamp
that has a small stream that flows in the Spring and when it rains
heavily.
There is a small swamp at the bottom of that gorge that is separated by
a low dike from the upper swamp. There is a culvert in the dike that
allows its water to flow into the upper swamp.
What I found on Saturday was that the lower swamp level had risen to the
top of
the beaver damn and there was an 18 inch wall of water flowing over the
submerged
beaver damn.
The water from the gorge from Millville Road was overflowing the dike
into the upper swamp. All the hiking paths around the upper swamp
were streams. There were streams flowing off the South ridges that I had
never seen in my 15 years of experience hiking those trails. I saw one
spillway that was a foot deep and 6 feet wide. Every 100 foot
or so along the South ridges there was a spillway flowing into the upper
swamp. During
the hour and a half that I was tramping through the area, the upper
swamp rose
better than 6 inches. I went in along the top of the dike and had to
find a new
route for my exit because by the time I returned, the water over the
dike was 8-9
inches deep instead of the 2-3 inches when I went in.
When I was returning, I checked the stream that is normally the outlet
from Lake
Nipmuc. What I discovered was that the stream direction had reversed and
was
flowing into the lake rather than draining the lake.
The high level of Nipmuc was the result of a combination of events.
First, the
lower swamp had filled and was restricting the normal outlet of the
upper swamp.
The upper swamp was undergoing flash flood conditions and filling from
the ridges to
its South and West and especially from the gorge coming from Millville
Road. This
overflowed into the lake and added to the level that had been retained
over the summer.
This level was the highest I have seen including during Hurricane Bob in
the
early 90's. From my observations, the primary cause of the flood was the
large amount of water flowing into the upper swamp and the fact
that the lower swamp was also rising, was like a stopper in an your bath
tub. With both the upper swamp overflowing into the lake and all the
other sources flowing into the lake, it was going to rise
until the rain stopped.
We started this with the lake full, the upper swamp full, and the lower
swamp
full. The lake and the upper swamp was full because the beaver damn
between
the 2 swamps was maintaining the level we saw this summer. I suspect
there is a
second beaver damn at the outlet of the lower swamp that was keeping it
full.
We haven't had beaver in the area since the 1700s. They have migrated in
from
Uxbridge in the last year. They will continue to re-populate the area.
Some residents have suggested that we can solve the problem by removing
the beavers or tearing down the dams. The gorge between the 2 swamps is
a natural choke point and if the beavers are removed, they will
re-populate it again and again.
They will populate Kathy Larson's swamp and also move into the gorge
that flows from
Millville Road into the upper swamp. There are small swamps up on the
ridges that will be also populated. Only extensive trapping will get us
back to where we were.
Mendon is accustomed to not having to deal with much flooding. This
recent 8 days
of solid rain is a wake up call. In the future we will have more stored
water
and we will have beaver dams collapsing. Lake Nipmuc is going to have
more high
water incidents. Residents who believe they can solve this by tearing
apart a single
beaver dam will get very tired of tramping through the swamps and
tearing up dams.
The beavers will win. They work 24 hours a day and 7 days a week. |