Saturday, July 21, 2018

Walden Pond - Water Level Changes

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.

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.

Circles show height of water measured in feet above 'sea level' (see scale on right side). The long-term average is
158 feet. Prolonged droughts cause dips. Extreme amounts of rain in a short period of time cause spikes.
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.

Kettle ponds are characterized not only by depth, but by clarity of water, the reason being that all the inflow is filtered. In contrast, a mill pond (or a beaver pond) will have silty sediment deposited by the erosive action of the dammed river. The later will support plants, and perhaps algae blooms, whereas the former – nitrogen and phosphorus poor – stay pristine. One of Waldon’s problems today is too many people peeing in the pond.* Please don’t!  

*   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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