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.