Septic Leach Field

MBTA Boston-Fitchburg Line (double rails)

Reservation Headquarters

Route 2

Municipal Landfill

Area

Former Trailer

Park Area

Emerson's Cliff

to Boston

to Fitchburg

Route 126

to Concord

Walden Street (Route 126)

Parking Areas

N

AQUATIC PLANT ZONE 

WALDEN POND RESERVATION 

EQUAL WATER DEPTH – in feet 

 below surface. Nominal elevation

 of pond surface is 166.5 feet 

 above sea level

EXPLANATION

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90

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Wyman's 

Meadow

Thoreau's Cove

Ice Fort Cove

Little Cove

PAVED ROADS

UNPAVED ROADS

HIKING TRAILS

0

0 100 METERS

500 FEET

Long Cove

Deep Cove

Boat Launch

Main Beach

Red Cross Beach

 Thoreau's 

original

cabin site

Walden Pond Bathhouse

42 ̊30'

0 50 KILOMETERS

0 50 MILES 

MASSACHUSETTS

73 ̊00' 72 ̊00' 71 ̊00'

70 ̊00'

41 ̊30'

Walden Pond Boston

USGS Fact Sheet FS-064-98

June, 1998

Figure 1. Walden Pond Reservation Map composed from Massachusetts Department of 

Environmental Management and U.S. Geological Survey sources.

Walden Pond, in Concord, 

Massachusetts, is famous among 

lakes because of its unique social 

history. Walden was the setting for 

American naturalist Henry David 

Thoreau’s well-known essay 

“Walden; or, Life in the Woods,” 

first published in 1854. Thoreau 

lived and wrote at Walden Pond 

from July 1845 to September 1847. 

In “Walden,” Thoreau combined 

highly admired writing on 

Transcendental philosophy with 

pioneering observations of aquatic 

ecology and physical aspects of 

limnology, the study of lakes. 

Because Thoreau also defended so 

effectively the value of living close 

to nature in the Walden woods, the 

pond is considered by many to be 

the birthplace of the American 

conservation movement. Visitors 

come from all over the world to the 

pond, which has been designated a 

National Historic Landmark, and 

its fame has resulted in a major 

fund drive to preserve the 

surrounding woods. 

Walden Pond has no surfacewater inflow or outflow, and much 

of its ground-water contributing 

area likely is preserved within the 

Walden Pond Reservation area 

(fig. 1). Only 15 miles from 

Boston, the pond is unusually clear 

and pristine for an urban-area lake. 

However, point sources of nutrients 

near the pond, and a large annual 

visitor attendance, concentrated 

during the summer when the 

swimming beach (fig. 2) is open, 

may contribute a nutrient load 

sufficient to change the pond 

environment. The occurrence of 

nuisance algal species, a recent 

beach closing, and an awareness of 

water-quality problems suffered by 

other ponds in the region raise 

concerns about the risk of 

ecological change at Walden Pond.

Despite the role of Walden Pond 

as a cultural and environmental 

icon, little is known about the 

pond’s ecological features, such as 

its internal nutrient cycling or the 

structure of its food web, nor have 

consistent measurements been 

made to determine whether these 

features are changing or are stable. 

Production rates of aquatic plants 

in lakes and ponds naturally 

undergo a slow increase as plant 

nutrients, organic matter, and soil 

U.S. Geological Survey

U.S. Department of the Interior
are leached and eroded from the 

surrounding watershed. This 

process is known as eutrophication. 

The term “cultural eutrophication” 

refers to an accelerated form of the 

natural process in which extra soil 

and nutrients are derived from 

people’s use of fertilizer, rerouting 

of surface drainage, and disposal of 

domestic and industrial waste. 

Cultural eutrophication can lead to 

excessive growth of aquatic plants, 

pond filling by decayed plants and 

eroded soils, reduced water clarity, 

and depletion of dissolved oxygen 

in deep water with subsequent loss 

of cold-water fish habitat.

In order to document the longterm ecological health of Walden 

Pond, the U.S. Geological Survey 

(USGS), working in cooperation 

with the Massachusetts Department 

of Environmental Management 

(MDEM), is investigating factors 

that could contribute to cultural 

eutrophication of Walden. Through 

measurements of mass balance of 

nutrients and oxygen in the pond’s 

deep water, the investigation will 

establish a baseline data set on 

Walden’s trophic state, which is a 

measure of the pond’s ability to 

support plant growth. The baseline 

data will be used to detect trends 

and give early warning of trophic 

changes or trophic response to 

pond remediation projects. 

This Fact Sheet provides 

background information on 

the environmental setting, 

limnological features, and cultural 

eutrophication of Walden Pond, 

and describes the joint 

USGS/MDEM study.

Environmental Setting and 

Limnological 

Features of Walden Pond

In geological terms, Walden 

Pond is known as a kettle lake. It 

was formed at the end of the last 

Ice Age, some 10,000 to 12,000 

years ago, when a huge block of 

glacial ice broke off and remained 

behind as the glacier retreated to 

the north. Meltwater streams from 

the main glacier carried sand and 

gravel and deposited this coarsegrained material around the base of 

the ice block. When the ice block 

finally melted, perhaps over a 

period of 100-200 years, the result 

was a steep-sided basin, or “kettle 

hole,” formed in the highly porous 

glacial outwash. The irregular 

shape of the pond, with its steep 

sides, coves along its margins, and 

two deep areas (fig. 1), reflects the 

shape of the original ice block. The 

maximum depth of present-day 

Walden Pond is 100 feet, virtually 

unchanged from the first accurate 

sounding by Thoreau 150 years 

ago. The pond can be described as 

a seepage lake, meaning that it has 

no surface inflows or outflows. 

Rather, it receives its water from 

direct precipitation and the inflow 

of ground water. Today, the pond’s 

ground-water contributing area, in 

the towns of Concord and Lincoln, 

is largely protected. Indeed, in 

Thoreau’s time it was one of the 

few wooded areas in a heavily 

farmed region. 

As first measured and discussed 

by Thoreau, Walden Pond (and 

other temperate-zone lakes) 

undergoes alternating periods of 

thermal stratification and mixing 

during the year. This pattern 

influences many chemical and 

biological processes that occur in 

lakes (Cole, 1983). During 

summer, the upper water becomes 

warmer and slightly less dense 

than the deeper water. As a result, 

only the water in the warm top 

layer (the epilimnion, fig. 3) 

circulates in contact with the 

atmosphere, whereas greater 

density keeps the deep, colder 

layer of water (the hypolimnion) 

away from the surface. Between 

the two layers is a zone in which 

Figure 2. Main swimming beach and bathhouse at the east end of Walden Pond.
MUD

SCHEMATIC ONLY, VERTICAL DIMENSION GREATLY EXAGGERATED

Epilimnion

Metalimnion

Hypolimnion

Pond Surface

ALGAL MATS

Ice

SUMMER CONDITIONS WINTER CONDITIONS

WATER TEMPERATURE, IN DEGREES FAHRENHEIT

DEPTH, IN FEET

DISSOLVED OXYGEN, IN PARTS PER MILLION

30 ̊ 40 ̊ 50 ̊ 60 ̊ 70 ̊ 80 ̊

0 5 10 15 100

80

60

40

20

0

OXYGEN

TEMPERATURE

WATER TEMPERATURE, IN DEGREES FAHRENHEIT

DEPTH, IN FEET

DISSOLVED OXYGEN, IN PARTS PER MILLION

30 ̊ 40 ̊ 50 ̊ 60 ̊ 70 ̊ 80 ̊

0 5 10 15 100

80

60

40

20

0

Epilimnion

Metalimnion

Hypolimnion

OXYGEN

TEMPERATURE

water temperature decreases rapidly 

with depth (the metalimnion), 

which is a barrier to mixing. 

Dissolved oxygen, which is 

essential to many forms of aquatic 

life, usually is abundant in the 

epilimnion, where it is produced 

during photosynthesis by aquatic 

plants and exchanges with the 

atmosphere during wind-driven 

circulation. The plants, which are 

the base of the aquatic food web, 

are grazed by zooplankton (fig. 4) 

and transported, in part as 

zooplankton fecal pellets, by 

settling to deep water. In the 

absence of light sufficient for 

photosynthesis, oxygen that was 

present in the hypolimnion when 

the lake became thermally stratified 

is consumed by the respiratory 

activity of bacteria and other 

organisms that break down organic 

matter raining down from the 

upper layer. 

With the onset of cool weather 

in fall, the temperature of the 

epilimnion drops until it is the 

same as that of the hypolimnion. 

At that point, the water is free to 

mix and cool further, drawing 

dissolved oxygen in from the 

surface to achieve a uniform 

distribution with depth (graph at 

right in fig. 3). Mixing continues 

until an ice cover forms, at which 

time the entire water body can be 

isolated from the atmosphere. In 

spring, when the ice melts, a 

second mixing period occurs prior 

to surface warming and the return 

of summer stratification.

Thoreau first recorded the 

clarity of Walden Pond, describing 

it and nearby White Pond as “great 

crystals on the surface of the earth, 

Lakes of Light” (Thoreau, 1854, 

p. 448). The extraordinary 

clarity of the water allows 

light to penetrate well into the 

metalimnion. Resulting 

photosynthesis in the metalimnion 

generates oxygen that can escape 

only slowly from the densitystratified water of that zone, 

causing a distinct metalimnetic 

Figure 3. Temperature and oxygen changes through the year in temperate deep lakes and ponds, modified from Odum, (1971). Summer profiles 

are from Walden Pond, July 22, 1996; winter profiles are typical of lakes under ice cover.

Figure 4. Zooplankton (Daphnia on left and calanoid copepod on right) grazing on phytoplankton 

(star-shaped Asterionella, and others) from the epilimnion of Walden Pond. Photomicrographs 

magnification about 130 X.
bulge in the summer dissolvedoxygen profile (fig. 3). In lakes 

with less clear water, summer 

oxygen concentrations decrease 

with depth. Much of the plant 

biomass producing oxygen in the 

metalimnion of Walden Pond may 

be the bottom-dwelling macroalga 

Nitella (fig. 5). In his “Journal,” 

Thoreau (1852) noted the presence 

of Nitella gracilis at depths below 

40 feet in nearby White Pond. The 

plant is also present at depth in 

other lakes celebrated for their 

clarity, such as Crater Lake, Lake 

George, and Lake Tahoe (Stross and 

Rottier, 1989). In Walden Pond, 

Nitella grows in a ring in the 

predominately blue-wavelength 

light that penetrates to depths of 28 

to 40 feet. Not present in less clear 

ponds, the alga may help preserve 

water clarity in Walden Pond by 

keeping oxygen concentrations high 

over large areas of the littoral 

sediments, which promotes binding 

of phosphorus by the sediments.

Cultural Eutrophication: 

Possible Causes and 

Consequences for 

Walden Pond

Many lakes surrounded by 

developed land receive excess 

sediments and plant nutrients (1) in 

runoff from agricultural fields, 

domestic lawns, and impervious 

surfaces, (2) by tributary and 

ground-water inputs of industrial 

and domestic waste, and (3) from 

precipitation and dry-fall 

atmospheric inputs. Although 

Walden Pond is in a developed 

area, the absence of surface-water 

inflow and the pond’s largely 

protected small ground-water 

contributing area decrease its 

potential nutrient load. 

Most precipitation that falls in the 

area that contributes ground water 

to Walden Pond moves through a 

thick unsaturated zone to the water 

table and then flows through the 

sand and gravel of the aquifer to the 

pond. Along this flow path, 

particulate nutrients are filtered out by 

the aquifer materials and much of the 

dissolved phosphorus, the nutrient 

that most commonly limits the growth 

of algae in ponds, is chemically bound 

to the aquifer materials. Phosphorus 

from point nutrient sources 

discharging to ground water, however, 

can exceed the absorption capacity of 

the aquifer, leaving a soluble residue 

that can move with the ground water. 

Walden receives no domestic or 

industrial waste directly from surface 

water, but the Concord landfill, a 

trailer park, and the septic leach field 

behind the Reservation headquarters 

are not far from the pond (fig. 1) and 

could conceivably contribute nutrients 

by way of the ground water. Direct 

atmospheric deposition of nutrients on 

Walden Pond may be substantial and 

should be investigated in light of 

recent findings that dry fall (“dust,” 

not precipitation) can be substantial in 

urban areas (Caraco and others, 1992; 

Kortman, 1991). Direct erosion of 

sediments and soil from the shoreline 

may cause some filling of the pond, 

but erosion is unlikely to be an 

important nutrient source because the 

sandy soil is probably nutrient poor. 

Biological sources of nutrients at 

Walden include wastes from 

waterfowl that roost on the pond 

surface at night, particularly 

during the spring and fall seasons; 

stocked fish; and people visiting 

the pond (swimmers). Nutrients 

from birds likely are decreasing 

since fill operations at the 

Concord Landfill ceased. The 

swimmer source of nutrients 

(urine) might be especially 

important because input takes 

place during the warm months, 

when thermal stratification keeps 

the hypolimnion from being 

replenished with oxygen from the 

surface. Walden receives about a 

half million visitors per year, 

concentrated during the summer 

months (fig. 6). The percentage of 

visitors who swim also increases 

on the hottest days during the 

summer. The swimmer source is 

unique in that it could conceivably 

be changed by management 

strategies, for example, through a 

swimmer education program.

Increased nutrient input to 

ponds generally stimulates plant 

growth and consequent oxygen 

depletion in deep water, but the 

suite of plant species stimulated 

and the effect on water clarity in 

specific cases are difficult to 

predict. Historical responses to 

Figure 5. Nitella scraped from the bed sediment with a weed rake.
JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC

0

20,000

40,000

60,000

80,000

100,000

120,000

NUMBER OF VISITORS 

MONTH

Epilimnion

Metalimnion

Hypolimnion

DISSOLVED OXYGEN,

IN PARTS PER MILLION

0 5 10 15

August 8, 1939

July 28, 1986

July 22, 1996

DEPTH, IN FEET

100

80

60

40

20

0

changed nutrient inputs can be 

inferred from comparison of oxygen 

profiles taken over time. The earliest 

available depth profile of dissolved 

oxygen concentrations in Walden 

Pond (fig. 7) was measured in 

August 1939 by Deevey (1942), a 

Yale University limnologist. Similar 

profiles were measured in 1986 by 

the MDEM, and in 1996 by the 

USGS. The average concentration 

of dissolved oxygen in the 

hypolimnion was 6.3 ppm (parts per 

million) in the August 1939 profile. 

In summer 1986, however, the 

average hypolimnetic dissolved 

oxygen concentration, 3.2 ppm, was 

one half of its 1939 value. The most 

recent depth profile shows the 

lowest average concentration of 

dissolved oxygen at 2.1 ppm. The 

progressive depletion rate of 

dissolved oxygen since 1939 is 

indicative of substantial 

eutrophication of Walden Pond, 

probably a consequence of 

increased loads of plant nutrients, 

particularly phosphorus. During the 

same period, oxygen concentrations 

increased in the metalimnion, 

probably because of increased 

growth of algal species such as 

Nitella. Metalimnetic algal 

production contributes oxygen to 

the deep water by vertical mixing 

because the water column is only 

weakly stratified at the bottom of the 

metalimnion. If nutrient inputs to 

Walden Pond continue to increase, the 

metalimnetic algal (and oxygen) 

production would at some point begin 

to decrease because of shading from 

increased planktonic algae in the 

surface water. The disappearance of 

oxygen from the hypolimnion after 

the decrease in metalimnetic 

production would be still more rapid 

because less oxygen would be 

contributed from the metalimnion. 

Present-day nutrient load is one 

measure of the trophic state of a lake 

or pond. A recent nutrient budget 

computed for Walden Pond indicates 

that 26 kilograms of phosphorus 

enters the pond annually (Baystate 

Environmental Consultants, 1995). 

This phosphorus load, which is 

difficult to measure directly, puts 

Walden in a higher trophic category 

than do more direct indices of trophic 

state, such as clarity of water and 

chlorophyll concentration (Baystate 

Environmental Consultants, 1995). 

When trophic state is evaluated by a 

fourth method, loss of oxygen from 

the hypolimnion (Wetzel and Likens, 

1979), Walden stands out as more 

eutrophic than other deep lakes in 

Massachusetts. A tentative 

reconciliation of these trophic indices 

(other than measurement error) is 

that Walden’s substantial nutrient 

input causes more growth of deep 

macroalgae, like Nitella, than 

would occur in most lakes with 

less clear water. Thus, the 

measures of lake clarity and 

chlorophyll, which reflect 

planktonic algal abundance, are 

not greatly affected by the nutrient 

input. The high rate of oxygen 

depletion in summer in the 

hypolimnion of Walden Pond may 

be related to the recreational input 

of phosphorus. Phosphorus input 

from swimmers was estimated to 

be about 25 percent of the annual 

phosphorus budget (Baystate 

Environmental Consultants, 1995), 

but would represent 50 percent of 

the summer budget. This would 

explain why Walden’s 

hypolimnetic oxygen depletion 

rate is higher than that of other 

Massachusetts lakes with fewer 

swimming visitors. These possible 

explanations need to be tested 

before being used to direct 

pond management. 

Figure 7. Oxygen profiles in Walden pond, in 

1939, (Deevey, 1941), 1986 (Massachusetts 

Department of Environmental Protection), 

and 1996 (U.S. Geological Survey).

Figure 6. Average monthly visitor attendance at Walden Pond, 1994–1996 (Massachusetts 

Department of Environmental Management).
USGS/MDEM 

Cooperative Study

The USGS and the MDEM have 

begun a study of cultural 

eutrophication in Walden Pond. 

The study is designed to establish 

the status of ecological features of 

the pond: to determine whether it is 

in a stable or changing tropic state, 

to determine the dynamics of 

nutrient cycles in the pond, and to 

quantify inputs from nutrient 

sources. Results of the study are 

expected to guide management 

decisions for the pond. Also, 

because of the pond’s unusual 

hydrologic setting—in a developed 

area but isolated from runoff—the 

data collected may be useful for 

assessing the effects of shoreline 

development, as distinct from other 

environmental impacts, on 

other ponds.

Trophic State Monitoring. A 

principal component of the study is 

to make consistent measurements of 

the pond’s trophic state by 

measuring oxygen depletion in the 

hypolimnion. This method is 

sensitive for deep lakes like Walden, 

which have substantial hypolimnetic 

oxygen reserves at the beginning of 

the year and thus are not subject to 

complete oxygen depletion before 

October, the end of the stratified 

season. Oxygen and temperature 

profiles will be determined monthly 

throughout the year to document 

how oxygen is generated or used in 

the pond and replenished or lost to 

the atmosphere. Particular attention 

will be paid to the oxygen content of 

the pond in early spring, which fixes 

the value for the beginning of the 

summer budget, and the late summer 

amount, which determines depletion 

for the entire season. By combining 

oxygen and temperature 

measurements, vertical mixing rates 

and fluxes of oxygen in the pond can 

be determined as well as rates of 

oxygen generation and consumption 

in specific zones of the pond. This 

monitoring will enable a clear 

assessment of the present trophic state 

of the pond, and if continued, will 

indicate whether the trophic state is 

stable, changing, or responding to 

programs of remediation.

Nutrient Dynamics. The basic 

limnology of the pond, including 

determination of the dominant 

planktonic and benthic algae, the 

biogeochemical cycling of nutrients, 

and the chemicals that interact with 

nutrients, is needed to determine 

whether Walden Pond might respond 

differently than other lakes to nutrient 

inputs. Profiles of the concentrations 

of silica, nitrogen, iron, manganese, 

sulfide, and phosphorus will be 

measured at two deep-hole stations on 

the pond. Additional water samples 

will be collected at one deep station 

and preserved for subsequent 

phytoplankton identification. Benthic 

aquatic plants will be assessed by 

divers, by suspending a video camera 

from the surface, and by trolling with 

a weed rake (fig. 5).

Nutrient Sources. Refinements to 

the estimates made for sources of 

nutrients from atmospheric 

deposition, swimmers, and ground 

water would be especially useful in 

managing Walden Pond. Swimmers 

represent a potentially large but 

controllable source, and ground water 

might be affected by point sources of 

nutrients from the nearby Concord 

landfill, the trailer park, and the 

septage leach field for the pond bath 

house. Swimmer input will be 

determined by measuring the increase 

of urine constituents in the epilimnion 

during the swimming season when the 

pond is stratified. Atmospheric input 

will be determined from samples 

obtained from a wetfall-dryfall 

collector. Nutrient contamination in 

ground water will be determined by 

defining exactly the ground-water 

contributing area for the pond and by 

sampling water from monitoring wells 

in background locations and locations 

that intercept any plumes originating 

from known point sources. 

References

Baystate Environmental Consultants, Inc., 

1995, Study of Trophic Level 

Conditions of Walden Pond Concord, 

Massachusetts: Massachusetts 

Department of Environmental 

Management, Division of Resource 

Conservation, 111 p.

Caraco N.F., Cole, J.J., and Likens, G.E., 

1992, New and recycled primary 

production in an oligotrophic lake: 

insights for summer phosphorus 

dynamics: Limnology and 

Oceanography, v. 37, p. 590–602.

Cole, G. A., 1983, Textbook of Limnology, 

3d ed.: Prospect Heights, Illinois, 

Waveland Press Inc., 401 p.

Deevey, E.S., Jr., 1942, A re-examination 

of Thoreau’s Walden: The Quarterly 

Review of Biology, v. 17, no. 1, 

p. 1–11.

Kortmann, R. W., 1991. Phosphorus 

loading - look to the sky: Lake Line, v. 

11, p. 19–20.

Odum, E.P., 1971, Fundamentals of 

Ecology, 3d ed.: Philadelphia, 

Saunders, 310 p.

Stross, R. and Rottier, B., 1989, Protecting 

water clarity in Lake George: The 

Conservationist, September-October, 

p. 44–46.

Thoreau, H.D., 1854, Walden; or, Life in 

the Woods, in Bode, Carl, ed., The 

Portable Thoreau: New York, The 

Viking Press, 1964, pp. 258–591.

Thoreau, H.D., 1852, Journal, Vol. 9, in

Broderick, J.C., ed., The Writings of 

Henry D. Thoreau: Princeton, 

Princeton University Press, 1981.

Wetzel, R.G., and Likens, G.E., 1979, 

Limnological Methods: Philadelphia, 

Saunders, 357 p.

For more information:

http://www.tiac.net/users/morganti/walden.htm

http://mass1.er.usgs.gov/proj.htm

U.S. Geological Survey

MA-RI District Office

28 Lord Road, Suite 280

Marlborough, MA 01752

telephone: (508) 490-5058

facsimile: (508) 490-5068

Report by:

 John A. Colman and Marcus C. Waldron

Layout and Graphics by:

 Mark V. Bonito and Robert F. Breault