Like many children, I suppose, I got my first experience of the implacability of nature on a beach. Mine was called Salt Creek Beach, a little curve of sunlit sand in Southern California bracketed at each end by a low headland. Each summer in the years after World War II, my family would join our tent to a Hooverville-like cluster of flapping canvas strung along the beach for 300 or 400 yards. For two weeks, I slept in a sleeping bag on the beach, explored the little creek canyon that cut through the uninhabited rise of land behind the tented conurbation, clambered over the rocks and waded among the tide pools in search of hermit crabs, starfish and sea anemones.

And built sand castles--intricate constructions with crenelated walls, guard houses at the four corners, drawbridges, interior rooms and windows, and, always, one dominant, sharp-peaked tower rising up from the complex, the kind of tower in which a long-haired princess might be kept prisoner.

That's how I remember those sand castles, anyway, and I'm sticking to my story, for any evidence to the contrary has long since vanished. Sooner or later, the waves would threaten my creation. Engineering then came into play. I would add another, deeper moat to encircle the one I had already dug. When that system began to fail, I erected long, V-shaped walls to divert water to either side of the castle. Nothing worked for long. Eventually, all I could do was stand back and watch the rising waters eat away at all the protection, pound against the walls of the castle, and, finally, topple the great tower into a pile of mush.

My boyhood willingness to challenge the inevitable day after day was surely forgivable. When adults do it, it is more accurately described as stupidity. Expensive stupidity, which has run into millions of dollars for castle builders and U.S. taxpayers alike.

Nothing more precisely illustrates the deranged conviction that we humans should be allowed to build and live anywhere we want to, logic be damned, than the compulsion to put our houses, and sometimes whole towns, where sea meets land in a conflict as old as time itself--one that the land is doomed to lose. We persist in defying this reality on all three coasts--Atlantic, Gulf and Pacific. By the beginning of the new millennium, roughly 80 percent of the U.S. population will live within an hour's drive of the coast. Yet the simple repetitive action of sea against land makes our seaside dreams as fragile as the sand castles of my youth.

On the outer reaches of Cape Cod, constantly moving dunes have drifted inland to cover streets and homes, and seasonal storm waters have floated houses off their foundations. From New England to Georgia, barrier islands get cut in half by storms and constantly roll over on themselves from wind and wave action to creep closer and closer to the mainland every year, making habitation a chancy business even in normal weather. Cliffs are undercut and eroded by the combined action of sea and rain, great chunks of their faces calving off like ice from a glacier--a condition especially endemic to Southern California, where television news reports regularly show multimillion-dollar homes sliding messily down to the rocks and pounding surf below. Most dramatically, from Cape Cod to Galveston, hurricanes have demonstrated our folly for generations, regularly blowing away whole neighborhoods.

If the only thing at stake were private property, it might be possible to take a philosophical view of the situation. But the costs of the National Flood Insurance Program--the only kind of insurance most beach-line dwellers can get anymore--and the Federal Emergency Management Agency expenditures that follow such disasters have become more and more of a burden to taxpayers. Hurricane Andrew in 1992, for example, did some $24 billion in damage, the worst of it in coastal regions of south Florida and southwestern Louisiana.

Beyond that, it is hard from an environmental point of view to accept the stubborn refusal of coastal residents to recognize that the sea eventually will have its way with them and all they have made. Like my frantic attempts to save my sand castles, the story of coastline development has been a tale of one frenzied attempt after another to halt, or at best, to postpone, the inevitable. The ultimate costs, in economic and environmental terms, are probably immeasurable.

But there are some clues. Seabright, N.J., stands as a paradigm, as Cornelia Dean shows in her somber study, "Against the Tide: The Battle for America's Beaches" (Columbia University Press). Situated down on the narrow shaft of Sandy Hook on the northern coast of the state, Seabright's beach was once hundreds of yards wide and ran for miles. As in many other regions, wealthy folks and developers found this stretch of uncluttered beach irresistible in the last third of the 19th century, and began building houses and hotels on the sands. These structures and a shoreline road impeded the natural movement of sand inland. The waves steadily ate away at the immobilized sand, soon threatening the road and the houses.

A seawall was built to protect them. By 1931, it was 12 miles long, 17 feet high and extraordinarily ugly--and it cost millions to maintain. Then the beach beneath the wall began to disappear, until by the 1990s there was nothing between the narrow little town and the sea during the high tide but the wall--and during storms, much of the town was inundated.

Then the citizens of Seabright turned to the federal government. In 1995, citing the need to reinstate eroded beaches to protect the town and sustain tourism, the U.S. Army Corps of Engineers began dredging millions of cubic yards of sand from the ocean bottom offshore and pumping it up to the beach. It was called replenishment, and while environmentalists and economic watchdogs in the Clinton administration decry the spending of millions of taxpayer dollars to protect beachfront property owners, there are plans under discussion that would replenish vanishing beaches along 130 miles of New Jersey coastline. Some say replenishment will have to continue regularly at least until 2045, at a projected cost of $60 million per mile.

"Unfortunately," writes Dean, "people in other states have failed to learn from New Jersey's sad example. By some estimates, almost 50 percent of the nation's shoreline is armored with hard structures and, despite new laws aimed at restricting them, they continue to proliferate, with predictable results."

The California Coastal Conservancy, a nonprofit group, tells me that in my home state many bluff-top homeowners have persuaded their local governments to install towering concrete cliff facings to halt erosion, at a cost of millions of dollars. These facings, like New Jersey's seawall, inevitably lead to beach erosion--exacerbating a long-standing problem. Most of the state's coastal rivers have been dammed for years, cutting off the coast's principal source of sand, which backs up behind all those dams. All that is left as a source are the naturally eroding bluffs, and as these are armored against their ultimate fate with facings, rock barriers and seawalls, more and more sand disappears.

As a result, virtually all of Southern California's recreational beaches now have to be artificially replenished. "So vast is the investment in these interventions," wrote Rasa Gustaitis in the Spring 1996 issue of California Coast & Ocean, the journal of the conservancy, "that some coastal experts have suggested that beaches in densely populated areas be thought of as part of the state's infrastructure, like water projects, roads, and utilities, rather than simply as natural features."

Endless care. All of this effort and all of this money, it should be noted, are not being expended to protect the land itself, in California or anywhere else that sea and land meet. These are not "conservation efforts." Left alone, the beaches and bluffs of coastline America could survive quite nicely, thank you, moving and reshaping themselves as the forces of the wind, tide and waves dictate in the ancient evolutionary saraband by which the coasts remain in a state of becoming. They do not need our protection. What they need is our absence. Yet we continue to erect our expensive barriers against the inevitable, trapped in our mad insistence that our elaborate castles, unlike those of my youth, are too precious to be sacrificed to reality.

T.H. Watkins, who holds the Wallace Stegner Chair at Montana State University, is the author of "The Hungry Years: A Narrative History of the Great Depression in America," to be published by Henry Holt in October.