By the time the Commonwealth acquired the property in 1922, much of Walden’s forest had been cut down. The woods have since grown back so that the vegetation resembles the hard and soft wood mix of Thoreau’s day and includes mostly berry bushes, sumac, pitch pine, hickory and oak. Above Thoreau’s house site are stumps of some of the 400 white pines planted by Thoreau and leveled by the great hurricane of 1938.

The wildlife of Thoreau’s time can still be found. Gray squirrels, chipmunks and rabbits are common. Skunks, raccoons and red foxes are active at night, but can occasionally be seen shortly before sunset or after sunrise. Kingfishers, blackbirds, chickadees and red-tailed hawks can often be seen flying among the trees or over the water. In the spring and fall, migratory ducks and geese pass overhead and land in nearby marshes for food and rest. As noted by Thoreau, the pond “is not very fertile in fish. Its pickerel, though not very abundant, are its chief boast.” The pickerel disappeared around the turn of the century and the pond is now stocked annually. In addition, sunfish, perch and smallmouth bass compete for crayfish.


Geology

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.


Wildlife

A variety of wildlife thrive today, including coyote and deer as well as porcupine, beaver, gray squirrel, red squirrel, chipmunks, turkey, woodpeckers (downy and hairy), red-tailed hawk, Canada goose, wood duck, mallard, black-capped chickadee, American robin, eastern phoebe, blue jays and cardinals.

Numerous fish swim the depths of Walden Pond.  They include: Rainbow trout, brown trout, brook trout, small mouth and large mouth bass, bluegill, pumpkinseed, rainbow smelt, koy, catfish and yellow perch.  The pond is stocked by Mass Wildlife in the spring and in the fall of the year.

Amphibians and reptiles make their home in Wyman and Heywood’s Meadow. These include: bullfrogs, gray treefrog, leopard frog, American toad, Woodhouse’s toad, painted turtle, snapping turtle, garter snake, northern water snake and eastern ribbon snake.