Of Tides, Mud, and Marsh

Ed Monnelly

More Information

Visit the Crane Wildlife Refuge. 

Doug Stewart is a journalist whose work has appeared in Smithsonian, Discover, Time, and Muse. This essay is excerpted from his introduction to Dorothy Kerper Monnelly’s book, Between Land and Sea: The Great Marsh, a beautiful photographic tribute to this stunning landscape. Ask for Between Land and Sea at your local bookseller.

This article originally appeared in the Summer 2009 issue of Special Places, The Trustees' member magazine. To subscribe, join The Trustees today.

“To sense the ebb and flow of the tides, to feel the breath of a mist moving over a great salt marsh, to watch the flight of shore birds that have swept up and down the surf lines of the continents for untold thousands of years, is to have knowledge of things that are as nearly eternal as any earthly life can be.”
– Rachel Carson

By Doug Stewart

A salt marsh is unique terrain. it’s not sea, yet  it isn’t quite land. Viewed from high ground on a stormy fall day, the Great Marsh in northeastern Massachusetts is one moment a parched-looking, windswept plain stretching to the horizon with only a tiny duck-hunting shack punctuating the distant flatness. A few hours later, if there is a spring tide (a twice-a-month occurrence), you’d think a catastrophe had arrived: the landscape is inundated, perhaps even topped with whitecaps and streaks of salt spray. Hedgerows of cordgrass barely perforate the surface of this ocean that has somehow materialized as if by magic (and which will soon vanish just as magically).

The drama, the immensity – the sheer sneakiness – of a spring tide flooding a marsh is always a surprise. At twilight with a full moon rising, you glance out a window at what you’re accustomed to thinking of as terra firma and are startled to see a trail of silver rippling toward the horizon. Under cover of darkness, the sea has crept in over the marsh.

The moon, aptly enough, is the key reason that salt marshes exist. Tides are regulated by the relative movements of the moon, sun, and earth, a celestial clockwork well outside the reach of human interference. The pull of the moon, and to a lesser extent the sun, acts on the earth to squeeze it slightly out of shape. Here and there, the oceans swell and drop slightly in response. As the earth spins, the continents are forever moving in and out of these bulges and dips, hence the tides.

Grasses that thrive under regular pickling in seawater are obviously well-adapted to what for other plants would be certain death. Relatively few are up to the task. This is one reason for the unbroken sweep of a healthy salt marsh: large swaths of it are dominated by a single species. On the New England coast, the low marsh, flooded at least half the time, is covered in smooth cordgrass, Spartina alterniflora. Anchored by a formidable network of roots, this sturdy grass can reach a height of ten feet. Slightly higher ground, which floods only occasionally, is blanketed in salt-meadow cordgrass, or salt hay (Spartina patens). At the upland edge of the marsh – drier and less salty still – salt-marsh elder and black grass dominate.

One of the most pristine salt marshes in North America, the Great Marsh extends along the coast of Essex County from Cape Ann to the New Hampshire border. It is the largest intact salt-marsh ecosystem north of Long Island, with more than 20,000 acres of largely undisturbed salt marsh, along with mud flats, estuaries, barrier beaches, and marsh islands.

Most of it is now protected as conservation land. In the flats along the tidal creeks and rivers, clamming remains a major commercial activity year-round; on the uplands, a few farmers still harvest salt hay each fall.

Biologically, marshes are the sun-fueled engine powering one of the most productive ecosystems on earth. A single acre can produce ten tons of organic matter in a year. By contrast, an acre of well-managed wheat field, laboriously cultivated and fertilized, yields a ton and a half.  The nutrients that marsh plants make available form the base of an astonishingly diverse food chain, from protozoa to mammals (including shellfish-loving humans).

Indeed, salt marshes provide sanctuary for an ark’s worth of wildlife. Tidal grasslands attract migrating waterfowl, shorebirds, and songbirds. Bitterns, or marsh herons, are so well adapted to marsh life, they disguise themselves when startled by pointing their bills skyward, impersonating stalks of cordgrass. Fiddler crabs scuttle among the grasses, scraping the mud for algae, while mummichogs and sticklebacks dart up and down the mud-walled creeks and channels, seeking food as well as refuge from deep-water predators. After the tides goes out, minnows and other small fish stay behind in the shallow pools, or salt pans, that interrupt the carpet of grasses.

A New England marsh’s subtle colors reward the patient observer. Sprays of salt-tolerant sea lavender, or marsh rosemary, reveal tiny purple-tinged blossoms along their branching stalks. On higher ground, black grass bordering the marsh is actually a rush related to the lily; its flowers are miniature lily blossoms. Thick-stemmed samphire, or glasswort, grows here and there in dense, ground-hugging forests; its scale-like leaves turn patches of marsh a flame-red in early fall. Even the dominant cordgrass, up close, reveals tiny purple flowers along its stalks.

Harder to miss are the dramatic shifts in a marsh’s character as the seasons pass. In spring, starting along the tidal channels, the grasses gradually shift from a muddy, monochromatic gray-brown to a softer, luminous green as energetic new shoots of Spartina begin to upstage last year’s brittle stalks. In late summer, the marsh is a radiant, wind-blown gold, as uniformly lush as a field of summer wheat. By year’s end, the marsh turns somber and austere once again as the Spartina dries up and dies back.

Even during a long New England winter, when much of the Great Marsh is as bleak as tundra, the surging tides animate the landscape. In cold spells, day and night, the tide lifts the thick brackish ice covering the channels, only to drop it again as it flows back out to sea. As the ice sheet falls, it pounds the frozen mud bluffs on each side of the waterways in imperceptibly slow motion. Massive shards of ice pile up in curving jumbles of debris, as though an earthquake had buckled the earth along an oddly winding fault zone. A few hours later, the swelling tide has stitched the rubble into a seamless plateau of white once again.


Over the past century, coastal states as a whole lost more than half their wetlands. Environmental regulations in recent decades have put an end to rampant salt-marsh destruction. Still, marshes continue to suffer unintentional damage. Because marsh grasses are adapted to such a narrow ecological niche, they’re vulnerable to human disturbances, like oils washed from roads into waterways and even nutrient-rich freshwater leaching from lawns.

The Great Marsh, healthier than most, is still vulnerable to development at its edges. More worrisome still, all barrier beaches are vulnerable to rising sea levels, which global warming is poised to accelerate. When the Ice Age glaciers melted and the seas rose, the marshes were able to keep up. If we allow the rate of sea-level rise to climb in the coming century, our marshes as well as our beaches will suffer. 

For champions of salt marshes, the most fundamental task is to educate the public about why these wetlands are valuable and how they can stay that way.