The origin of Walden's water was a mystery until the author proved a surprising source. By Eugene H. Walker, a long-time Concord resident and geologist. Excerpted from "Man and Nature", a former publication of the MA, RI, and ME Audubon Societies, and the CT Conversation Association, December 1971, pp. 11-20.

Popular belief has held for a long time that Walden's [water level] fluctuations are not related to local weather and, therefore, are unpredictable and mysterious. Generations of fishermen have noted that Walden does not rise with each rain and fall with each dry spell as do other ponds in the area, especially Sandy Pond a mile to the east.

Thoreau remarks on this matter, observing in his Journal (August 19, 1854) in the dry summer of 1854 that Sandy Pond was very low with the shore "...so exposed that you can walk around, which I have not known possible for several years, and the outlet is dry. But Walden is not affected by the drought."

Those who wondered why Walden does not seem to rise and fall with local weather long ago concluded that the source of Walden's water lay somewhere outside this locality. This theory has been strengthened by the facts that has no inlet, and is fed by springs whose source no man can see, and also by the common opinion that the slopes around it make a watershed too small to supply so large and deep a pond.

Over the years, the speculation that it is fed from a source far away has gradually hardened into an accepted fact and has become embroidered with details. Tales are told around town of the hole in the bottom of and the stream that comes through it, connected perhaps to a river that is rumored to run underground from somewhere in the White Mountains, perhaps Lake Winnipesaukee, southward to Cape Cod.

"Many Pretend to Know"
The mystery that surrounds has been deepened because no one has really known the pattern of rises and falls, for lack of systemic measurements. Even though Thoreau lived by the pond for two years and visited it many more, he knew little about the matter, summing up what he did know in his book -- "The pond rises and falls, but whether regularly or not, or within what period, nobody knows, though, as usual, many pretend to know."

Someone of those who pretended to know invented the theory that it rises slowly for seven years and then drops abruptly because it is drained by a siphon. This idea, too, can be heard around town, and has been attributed to Thoreau.

The most significant thing that the record shows is that the water level of Walden rises and falls through seasons and years almost exactly in agreement with changes in local ground-water level. It is like a very large well surrounded by water-saturated sand and gravel, and its water level rises and falls with the changes in water level in the sand and gravel.

Glacial Origins
Walden occupies what the geologists call a glacial kettle. About 10-11 thousand years ago a very large block of ice lay melting there, a short distance south of the main body of the shrinking glacier. Torrents of meltwater from the ice to the north spread sheets of sand and gravel around the isolated block for many years before the currents turned elsewhere. Finally, the block of ice melted and left a pool of water surrounded by banks of sand and gravel.

This origin makes it unlikely that water flows up a hole in the bottom of the pond. There probably are a few feet of sand and silt on the floor of and underneath lies the hardpan or glacial till that the glacier spread, and deeper is the bedrock that crops out in ledges around town. The bedrock and hardpan are the poorest water-bearing formations in this region and yield only a few gallons of water per minute to wells. The sand and gravel is highly pervious and yields hundreds of gallons of water per minute to a large number of town and industrial wells. So it is much more reasonable to suppose that Walden is fed by seepage from sand and gravel than through unlikely passages in bedrock and hardpan.

Is the Walden Watershed Sufficient?
One of the reasons behind the idea that Walden is fed from afar is the conviction held by many people familiar with the locality that the slopes around Walden could not provide enough water to keep Walden properly supplied. Even Thoreau held this idea, for he wrote (Journal, August 27, 1852) that "The water shed by the surrounding hills is insignificant in amount," and suggested that the slow rises and falls of Walden were due to changes in the amount supplied by the deep springs fed from some unknown source.

Is it really true that Walden's watershed supplies only an insignificant amount of water? Some of those who have repeated this statement may have been deceived because no brooks run into Walden. But there is a good reason why no brooks run into Walden. Water from rain or melting snow sinks into the sandy soils of Walden's watershed before it can gather into rills and brooks, as anyone can see if caught out in a heavy rain. Just as much water reaches Walden by seeping underground as by surface runoff -- perhaps more because by moving underground it escapes evaporation. Even through one cannot measure the invisible seepage, as one can measure the flow of a stream, a little figuring shows that the contribution from Walden's watershed is more than many people might think.

[After doing the calculations (see them here), i]t is...easy to see that the sand and gravel around Walden act as a large equalizing reservoir, and this explains the old puzzle of why Walden does not rise or fall with each storm or dry spell, as does Sandy Pond. The watershed of Sandy Pond is mostly glacial till, which sheds more water than it absorbs, so that much of a hard rain runs off quickly to raise the level of the pond. The same rain on Walden's watershed sinks into the sandy soil and causes a rise so delayed and relatively slight that it normally escapes notice. Sandy Pond declines notably in dry spells because there are no large bodies of water-bearing sand and gravel in its watershed to feed seepage into it between rains.

The result of this examination of water levels and rainfall and the figuring may perhaps change the image of Walden. For a pond that rises and falls in an unpredictable way, perhaps because fed through a hole in its bottom that is connected with an underground river form Lake Winnipesaukee in the White Mountains, we have substituted a pond fed by seepage form the sand and gravel that enclose it, that rises each year in the season of replenishment of ground water and falls in the season when growing plants use all the rain, and fluctuates more widely with groups of wet and dry years.

The Lure of a Good Mystery
Mystery exerts an attraction that people often regret losing -- one remembers how the poet Keats complained that a rainbow never meant as much to him after he heard someone explain how it was formed by raindrops bending light rays. Walden can never have for us the magical quality it possessed for those who dreamed that it had no bottom and extended indefinitely down into cold darkness. For such lost magic and for dreams of water-bearing tunnels from the White Mountains we have to substitute the satisfaction of knowing that Walden rises and falls, as if breathing, in perfect harmony with the surroundings that supply its water.