|Walden Pond is a kettle hole formed by
the Ice Age
Walden Pond is a kettle hole, a deep pond formed
about 15,000 years ago when the last glacier to cover New England
slowly melted away. As it did, a large block of ice broke
off into glacial Lake Sudbury from the retreating glacier.
The ice block eventually was surrounded by sediments ranging
from fine sand to coarse gravel deposited by streams
flowing from the glacier. As the block melted, it left behind
an indentation that eventually filled with water.
The pond has three deep areas. The maximum
depth (over 100 feet, or 30.5 meters) essentially is unchanged
from measurements made by Henry David Thoreau in 1846. Walden
Pond has no streams flowing in or out: it gains water
from the aquifer along its eastern perimeter and loses water
to the aquifer along its western perimeter.
Walden Pond potentially is threatened by environmental stresses
common to urban lakes: a municipal landfill, septic leachate,
high visitor-use rates, acid and other contaminants from atmospheric
deposition, and invasion of exotic species. Walden Pond retains
clear, undegraded water because of conservation efforts that
protect the shore and woods surrounding the lake.
A significant portion of Walden’s current plant growth
below depths of 19 to 41 feet is Nitella, a large
alga often associated with clear-water lakes. To survive, this
plant requires deep light penetration. By tying up nutrients
at the sediment-water interface, along the pond’s bottom,
this plant keeps nutrients away from potential algal blooms
at the surface.
One possible source of nutrients for Walden Pond and most other kettle-hole lakes
in eastern Massachusetts is swimmers. Large numbers of swimmers, estimated at
220,000 per summer, are not a new circumstance for Walden Pond.
In 1935, the Concord Herald reported that summer Sunday
afternoon crowds reached 25,000, and
that total summer attendance was 485,000. During his weekend measurements in
1939, Edward Deevey of Yale University counted “nearly 1,000 bathers.”
Management of water quality at the Walden Pond
State Reservation focuses upon maintaining the transparency
of the surface water so that the deep-growing Nitella can
continue its role in maintaining water clarity.