Imagine children at the seashore, digging a hole in the
sand. Water quickly seeps in until the water level in the hole matches the
ocean. Dug during a falling tide and the water in the hole will drop and may
even go dry, only to refill hours later, with the rising tide. Writ larger and
slower, this helps explain the rise and fall of the water level of Walden Pond.
In Henry David Thoreau’s time and ours, the water level of
Walden Pond has a long-time average of 158 feet above sea level. What the
occasional visitor misses – what the regulars well know – is that the pond can
range as much as four feet above or below that average. Current conditions are
at the low end of the range. The barren space between the treeline and water’s
edge is wide.
Thoreau wrote that as a child of four, his family had picnicked at Walden, making a beach fire on a sandy spit of land not far from where he would build his cabin for his two-year sojourn in Walden Woods. Early in 1846 he drilled more than 70 holes in the ice in an attempt to locate the pond’s deepest point. He reported depth at 102 feet, but to today’s topic, the picnic location was at that time under seven feet of water.
Thoreau wrote that as a child of four, his family had picnicked at Walden, making a beach fire on a sandy spit of land not far from where he would build his cabin for his two-year sojourn in Walden Woods. Early in 1846 he drilled more than 70 holes in the ice in an attempt to locate the pond’s deepest point. He reported depth at 102 feet, but to today’s topic, the picnic location was at that time under seven feet of water.
Walden Pond is a kettle hole pond. There are many scattered
across eastern Massachusetts, especially on Cape Cod. Envision the area now
identified as Walden Woods being filled by gravel and sand, deposited during the
melting of ice age ice some 18,000 years ago. Geological maps show this as the
remains of Glacial Lake Sudbury. Where the pond is now was occupied by a
massive, slower-to-melt mound of ice, so that when it finally melted, it left a
deep depression in the alluvial plain which stayed filled with water. Voila!
Walden Pond!! Kettle hole ponds typically have little to no surface-water
inflows or outflows. Instead, they receive all of their hydrologic inflows from
groundwater and precipitation.
On average, the pond gains about 20 percent of its total
volume each year from rainfall, snow melt, and groundwater seepage from the
east, losing the same volume to evaporation plus subterranean seepage to the Sudbury
River, to the west. There is a yearly cycle of water level change of about one
foot, highest after snowmelt, lowest at the end of summer. Drought years and
rainy years have a greater effect on the pond, but slower, to reflect the slow
movement of groundwater in and out of the pond. For many years the U.S.
Geological Survey measured groundwater levels in a well near Walden Pond, data
expressed as feet above sea level. The recorded low was a consequence of the
drought of 1964-65. Spikes in 1984 and 2010 represent the acute impact of
severe storms – in June 1984, 5-9 inches of rain in a few days, and in March
2010, 15 inches of rain from a series of storms that also caused the last major
flooding of the Assabet and Sudbury rivers. Present day, Walden Pond is below
its long-term average.
From Boston’s weather records, over the past 120 years the
region has become warmer, resulting in shorter winters and drier summers, but
annual precipitation has significantly increased. Some wetlands have expanded,
resulting in drowned forests, i.e., groves of dead but still-standing trees. However, it is hard to predict how climate will change in
the future, or how those changes will impact Walden Pond.
*http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0191755 Stager JC, et al. Climate variability and cultural eutrophication at Walden Pond (Massachusetts, USA) during the last 1800 years PLoS One. 13(4): e0191755.
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