Of Lake Cochituate's chain of
ponds, traditionally only the South Pond permits water-skiing. Just
north of the southeast
corner of Middle Pond, boaters heading for South Pond turn into a small
cove. Once inside this cove, one proceeds to
the triangular, slightly-arched tunnel
opening - and hopes the boat will fit through.
Canoes and kayaks have no
problems, but many power boaters face a tight squeeze.
How tight? So tight, that boaters have given it a nickname, "The Keyhole". So tight, that boaters slowly hand-over-hand past the tunnel's granite-block walls (as Natick's national water-ski competitor, Than Bogan, demonstrates here).
So tight, that it keeps
the big boats out. And when the water
rises a few inches, this triangular tunnel grows tighter yet. So tight,
that some boats have their windshields fastened down using wing nuts,
for easy removal when the lake's water level is high. Occasionally a
boater may pull the drain plug and flood the floorboards, to drop the
few inches. Once through the tight tunnel and back out into the bright
daylight, running the bilge pump
bails the water out of the boat. Now that's tight!
Although the height of
Lake Cochituate is regulated by the dam
on North Pond, Cochituate State Park staff cannot lower the lake
much more to cut the boaters more slack. Doing so would cause many
boats to run aground on the shallow tunnel bottom.
Luckily for boaters, the
Park staff can't raise the lake more,
either, because the dam
has an important public-safety function: flood control. Staffers
watch the weather predictions, to provide temporary holding capacity in
the lake. This delays the peak of stormwater runoff from Lake
Cochituate and the 27-square-mile watershed (drainage basin) that feeds
it. That delay, in turn, removes its contribution from the peak of
flooding on the Sudbury and Concord
Surprisingly, this tight tunnel once was extremely spacious. In 1846, when Long Pond was engineered into Cochituate Reservoir, Boston's first public drinking-water supply, a North Pond dam (probably not its first) was built to raise the lake's surface an additional nine feet. At this Middle Pond Tunnel, the new Saxonville Branch Railroad's granite bridge arched well above the lake's blue water. The tunnel interior was a graceful ogive arch.
Detractors had said that
Cochituate Reservoir was too grand a
design, too far from Boston, and too expensive. They said that Boston
wouldn't expand to need that much water capacity for at least two
hundred years! But within twenty years, engineers were scrambling to
more water. They built a new dam, four feet higher. The North Pond Gate
House had to
be disassembled, granite block by granite block, and reassembled along
the raised shoreline. Over a century later, you still can see the
contrast between the older stonework along the lake edge there, and the
newer stones higher up that retaining wall.
And that's the reason for
Lake Cochituate's tightest tunnel.
The four feet that raised the water were four feet removed from the
tunnel's headroom. The wide, parallel walls now lie
hidden, far below where boats squeeze ever so carefully through the
narrow top of that lovely ogive arch.
[Later: Add original tunnel arch blueprint, here - from MWRA Archive, 070807?]