In the warm, wet sand of late spring, on a tidal southern Maryland river, delicate four-clawed prints emerge from the water, heading inland. Several yards down the shore is an identical track going back. Where they converge in a thicket of beach grass, the smooth sand offers no clue anything ever happened. But gentle probing reveals a freshly laid nest of leathery, cream-colored diamondback terrapin eggs.

Most of these will hatch out male, because this beach faces north and east, and its incubating sands remain relatively cool. A nearby south-facing strand, hotter, will hatch females.

The next beach downriver will hatch nothing. A homeowner, worried by minor erosion, has rip-rapped his banks solidly with stone, obstacle enough to ensure that female terrapins who may have been returning to this same spot to lay eggs for 20 years or more, never will again.

* * *

In June, a full moon bathes the fringes of an Eastern Shore marsh. As the tide nears its zenith, dark, horse-hoof forms glide laterally back and forth in the sandy shallows a few feet offshore. Periodically, one intercepts a larger version of itself heading purposefully for the bank. The smaller male horseshoe crab clasps to the rear of the big female, who hauls him ashore to spawn.

The ancient crabs, actually more related to spiders, have done this annual dance since a hundred million years before the dinosaurs appeared. “Cruisin’ for chicks” a biologist once described the moonlight patrols of the males, surely an enduring pastime.

* * *

July morning sun brings a fresh breeze as it rises over Smith Island in mid-Chesapeake. The wind and chop have riled the shallow bottom, muddying the Bay—except for a couple of acres where the water beneath my kayak remains calm and clear, its surface glistening green, flecked with gold in the long slants of early light.

I’m gliding over a bed of widgeon grass, one of the Bay’s dozen or so varieties of submerged aquatic vegetation, bursting with seed that resembles tiny kernels of ripe corn. In less polluted times, the calming grasses grew thickly across hundreds of thousands of acres, suppressing shoreline erosion.

Look down as the wind scoots you across a magical world in the meadow below. Blue crabs lie mating. A flounder startles from a clearing. Showers of silver minnows fling skyward, followed by the great, swift bulge of some predator moving just under the jungle canopy.

* * *

Savoring steaming morning coffee, I walk to where a tidal Bay river passes my front yard. The dark smooth water reflects the harvest moon’s ghostly setting, framed by scarlet leaves of a black gum. A few stems of three-square reed poke from the water’s surface, adding a serene line to the composition. Beneath the mirrored moon, a sunlit seagull flaps across the reflection. A universe in a few square feet.

* * *

Tide softly fills the little marsh that fringes the shore near the river’s meeting with the Chesapeake. Sparrows sing, fish swirl, the noon sun gleams among the roots of the flooded spartina grass.

You can’t hear it or see it, but this marsh is doing serious work, furiously converting waterborne nitrogen, the Bay’s biggest pollutant, carried downstream from sewage, farm fields and dirty air, into harmless gas that will go back into the atmosphere.

Such marshes occupy only 1.3 per cent of the river’s basin, yet scientists calculate they cleanse the Bay of nearly 2 million pounds of polluting nitrogen every year on average. To do the same with sewage treatment plants would cost millions annually.

And this is just from one tiny subset of the Bay’s tidal wetlands. I’d be tempted to say this is valuable, but nature sends no bill for its services, and when we destroy a marsh we often don’t understand all we’re losing.

* * *

In the warm, wet sand of late summer, on a tidal, southern Maryland river, the faintest of imprints, twin treads perhaps an inch apart, with a solid line in between, signifies where a terrapin hatchling, dragging its tiny tail, crawled safely from nest to Bay, to start a life that may last half a century.

* * *

A theme connects all the above, threads through much of what is fine and essential about the Chesapeake estuary, emanates from a fundamental quality of nature. It has to do with edges. Life loves those joints, intersections, overlaps and seams where land joins water, forest borders field, fields fall off to beaches and marshes, marshes to shallows, shallows to channels.

When ecologists and economists recently attempted to calculate the value of all the Earth’s natural resources, the astounding thing was not that it was in the tens of trillions of dollars—rather that nearly half of it came from 10 percent of the planet—the edges—wetlands, aquatic grass habitats, coral reefs, the rich fishing grounds of the continental shelves.

Such fecund junctures of land and water our Chesapeake has in hyperabundance. My office wall is dominated by a 5-by-3 map, Landforms and Drainage of the 48 States. In striking silver and black, it shows the geophysical nation, no roads, no cities, just rivers and mountains and coastlines.

Follow the continental edge, and almost immediately one’s eye goes to the region where the 200-mile long Chesapeake and its dozens of tidal tributary rivers stab deeply inland, draining close to a fifth of the entire East Coast. From Puget Sound to Galveston, from Mobile Bay to the Outer Banks to Long Island, Cape Cod and the Gulf of Maine, there is nothing else like it.

The official length of the land-water edge created by these meanders, marshes, islands, peninsulas and coves is said to be more than 8,000 miles—I have seen estimates to 11,000 miles. I think either number—any number—understates the implications of the Bay’s edges. It has to do with how you measure.

I enjoy taking the edge’s measure from a 17-foot long, 23-inch wide kayak. It’s a craft superbly fit for the task, able to traverse the Bay’s broad, open waters with aplomb, yet poke into every nook and cranny, skimming the estuary’s astounding shallowness—only 22 feet deep on average, although it measures roughly a million feet long from Havre de Grace to Norfolk, and a 100,000 feet at its widest.

Often, I’ll paddle for 100 yards or so along a certain marshy edge in a few minutes, trolling for 2– to 3-pound striped bass feeding under the banks. The stripers, even when they grow to several feet and to 75 pounds or more and begin migrating to and from their natal Chesapeake, still hug the edges, and are a favorite of surf casters and coastal fishermen from Maine to the Carolinas.

Later, when the tide is up, I’ll retrace my short route, nosing into small creeks that rive the edge. Many of these branch, and many branches open farther into expansive, hidden ponds walled by tall marsh grasses, elegant ‘rooms’ known only to periwinkles climbing the stems of the grass, minnow-stalking egrets, dabbling ducks and blue crabs half-buried in the muddy bottoms.

My hundred yards of edge, traversed in minutes, has blossomed into miles, and a few hours’ adventure; and if my kayak and I could become smaller and smaller, the length of that edge and opportunity for exploration would grow almost infinitely.

Map makers have long pondered this mystery and fascination of the edge, which grows longer and longer as we measure it more closely. Indeed, a branch of mathematics known as fractals has grown up around the phenomenon of finite areas, whether the continental United States or Smith Island in the Chesapeake, being bounded by an infinite line.

Fractals are used to comprehend complex patterns and solve problems in fluid dynamics, and I don’t really understand them. But I do understand that the pleasures and fruits of the edge are virtually boundless:

They include sunsets on sand dunes that tower to 80 feet on Virginia’s Eastern Shore, and sunrises that light the fossiled cliffs of Calvert in southern Maryland like desert redrock. Also, sweeps of needlerush and spartina grasses that spread prairielike, changing color and texture with every shift and caprice of the light and the wind and the seasons. Up Bay rivers, where saltwater turns to sweet, stands of tall, plumy, wild rice cloak the banks in lemony-gold, and roar with the sound of redwinged blackbirds feeding there by the thousands.

Eagles and ospreys, bouncing back strongly now that the more noxious pesticides have been purged from the ecosystem, increasingly adorn the forested edges with their nests. To watch them soar and sky dance and plummet on fish is to know why raptor and rapture spring from the same Latin root.

Spring comes to the Bay’s edges with a flourish—blossoming shadbush, fringe tree, swamp azalea, hibiscus and viburnums—and the fresh, gauzy greens of bald cypress, the red blush of swamp maples that begins as early as February. Smells seem particularly pungent in the moist air along the edge—sweet locust blooms in May that signal the crabs are beginning to shed, as well as the perfumes of clover-scented black cherry and creamy wild magnolia flowers.

Because of its essential shallowness, the Bay’s edges are not only long, but very broad—about 20 percent of the whole place is no more than a couple of meters deep. This lets sunlight penetrate, growing underwater grasses, havens for crabs; as well as fueling plenty of planktonic food for oysters. The very shapes of watermens’ boats, from soft crab scrapers to oyster skipjacks, evolve from working the long, shallow edge.

An emerging theory credits a more ancient edge with enabling the settlement of America by peoples from western Europe even before humans came across the land bridge between Asia and Alaska. Recent finds of native tools in the Chesapeake region match artifacts used in Europe around 13,000 years ago.

But how could primitive people sustain themselves on such a sea voyage, and what would have drawn them into a perilous void like the North Atlantic?

At the time, ice probably clogged the Atlantic down to a latitude about that of the present-day Bay; and edges of ice and open water are among the richest of environments. Minerals from melting glacial ice support a fantastic abundance of mammals, fish and birdlife that might have been hunted all along the way.

Indeed, humans as well as nature have always loved the edge. Half of all the people on this earth are estimated to live on about 5 percent of the planet—and most of that chosen space happens to be on or near coastlines and rivers. More than half of 300 million Americans live within an hour’s drive of coastal edges (including the Great Lakes), and more are moving that way every day.

And why not? A few years ago a small group of us, who never miss a chance to worry about the Bay’s decline, gathered for dinner on a knoll overlooking a wooded cove that opened out to the main Bay. We boiled sweet corn and sliced tart, juicy tomatoes grown in fields behind us. We steamed fat, hard crabs and grilled fresh filets of catfish taken that day from the water in front of us. We washed it all down with a local white wine and savored a cooling breeze as sunset disgorged colors onto the cove’s waters.

Scientists, educators, journalists—we knew the water out there had its problems…but praise the crab and pass the wine. There on the edge, life was good.

You need times like that to recharge, to remember that while we’ve lost lots, a lot remains. Still, there’s growing evidence that if we truly love our Bay edges, we must show it by keeping our distance.

Scientists at the Smithsonian’s research center on the Chesapeake are documenting that the estuary’s natural edges begin declining even at very low levels of human disturbance.

Developing even 10 percent of the land around a given section of tidal shoreline can raise toxic PCBs in resident fish by several fold. At little as 14 percent of development triggers dramatic declines in marsh-dwelling birds like rails and herons.

Buffers of natural vegetation as wide as 3,000 feet—more than half a mile—may well be needed to fully protect the edges from our developments, the scientists think.

For the most part, that ain’t happening. Indeed, one can make the case that we are, in effect, hardening our hearts toward the natural, woody, marshy, sandy order of the Bay watershed.

Acres paved over throughout the watershed in the last decade or so have been increasing at a rate five times as fast as population is growing. This greatly increases the runoff of contaminated stormwater, one of the Bay’s fastest-growing sources of pollution.

And the edge itself—it is apparent on virtually every kayak journey I make—is becoming more hardened every year. Stone riprap and the sheer walls of bulkheads have wiped out between 400 and 500 miles of natural tidal shoreline in the last decade or so. Between Maryland and Virginia, we’re on a pace to armor about 4,000 miles of edges in the next century to protect private property from erosion.

There are often good alternatives to such armoring, like constructing wetlands or offshore breakwaters; but they are scarcely the norm.

There is nothing fractal, no infinity of nooks and crannies about an armored shoreline, nor much of any tidal wetland there to cleanse pollution from the Bay. No terrapin or horseshoe crab crawls ashore to lay its eggs. For the most part, there aren’t any overhanging branches to frame the moon’s reflection.

Strangely enough, looking out over your bulkheaded lawn at sunset on the Bay, drink in hand, eating steamed crabs and fresh fish filets, life can still be pretty darn good. More so if you don’t know what used to be there.