In spring 2016 Walden Pond is low, exceptionally so. Visitors are enjoying the broad expanse of sandy margins around much of the pond; they can walk and play without being confined to the trails and main swimming beach. This low water represents the extreme end of a drawdown that has been happening (irregularly) since the pond’s recent high in the spring of 2010, when water entered the forest around the pond and even flooded the paths. The contrast between these high- and low-water years is striking.

These changes in water level, however, are not new—they are one of Walden’s endearing quirks. As Henry David Thoreau noted 160 years ago in Walden: “The Pond rises and falls, but whether regularly or not, and within what period, nobody knows, though, as usual, many pretend to know. It is commonly higher in the winter and lower in the summer, though not corresponding to the general wet and dryness.”

Walden Pond’s ups and downs dictate much of the ecology of the pond. High water strangles encroaching trees and shrubs, helping to keep the pond margins open. Thoreau noted in his journal, “This great rise of the pond after an interval of many years, and the water standing at a great height for a year or more, kills the shrubs and trees about its edge—pitch pines, birches, alders, aspens, etc.—and falling again, leaves an unobstructed shore.”

The past few years of low water have allowed alder and birch saplings and wildflowers, such as lance-leaved violets and yellow wood sorrel, to establish on the sandy beaches below the forest margin. Many nonnative plants have also established, including coltsfoot, vetch, and dandelion. When the water rises again in coming years, this invading edge of plants will be drowned.

The water has fallen so low this year that part of a submerged sand bar—one that Thoreau described in his survey of Walden Pond—is now exposed. In Thoreau’s 1846 survey, he noted that the sandbar across the mouth of what is now known as Thoreau’s Cove on the northwest corner of the pond (the cove closest to the site of Thoreau’s cabin) was 7 feet below the water’s surface; water was 28 feet deep on the pond side of the bar and 20 feet deep on the cove side. Right now the sand bar extends above water from the eastern edge of the cove pointing toward the railroad.

Also related to the low water level in Walden Pond, the large vernal pool, known as Wyman’s Meadow, has no standing water. The meadow is separated from Thoreau’s Cove by a higher sand bar that is covered by a boardwalk. In typical years, the meadow fills with water in the winter and spring to a depth of around three feet. In the spring, the water is full of gelatinous egg masses from mole salamanders, American toads, and other amphibians; later in the spring and early summer the pool is alive with thousands of tadpoles and juvenile amphibians. By midsummer, the pool dries out and the meadow becomes a carpet of flowers—pink-flowered meadow beauties and sprays of pink and white knotweed flowers. But this year, without any standing water, the amphibians that call this pool home have no place to breed—and children visiting Walden will have no tadpoles to catch. In the wet spring of 2010, the amphibians of Wyman’s Meadow had the opposite problem. The water rose above the protecting sand bar and connected the meadow to Walden Pond. Hungry fish from the pond likely entered the meadow and ate the eggs and tadpoles.

The irregular cycle of high and low water creates a cycle of winners and losers, good years and bad years, depending on your perspective, whether you are a colonizing plant, a breeding salamander, a hungry fish, or a visitor enjoying the beach. The extreme low water and wide beaches that we are seeing this year—an extreme that Thoreau noted, too—is a part of what makes Walden Pond so endlessly fascinating. Something to consider next time you take a stroll around the pond.

Richard B. Primack is a College of Arts & Sciences professor of biology and the author of Walden Warming: Climate Change Comes to Thoreau’s Woods (University of Chicago Press, 2014). For the past 14 years, Primack, along with his students and colleagues, has been investigating the effects of a warming climate on the plants and animals of Concord, building on the observations of Henry David Thoreau from the 1850s. Primack can be reached at primack@bu.edu.

A version of this article was originally published in Wicked Local.