AMERICANS' love affair with the seashore is pitting them against nature's strongest force -- in a losing battle. The battleground is the 300 or so barrier islands that stretch along the Atlantic and Gulf coasts from Maine to Texas. Composed mainly of shifting sand, the barrier islands are in constant motion. Normal waves, storms and the slowly rising sea level push the islands toward the mainland, while other wave forces move them lengthwise along the coast. Left undisturbed, the constant play of wind and tides moves and reshapes the islands with little damage to the natural system. Indigenous plants and animals are adapted to constant change and movement. Invaluable salt marshes and estuaries -- whose existence depends on the protection afforded by the barrier islands -- continue as productive fisheries.

The problems begin when bridges, highways and houses -- none of them adapted to movement -- begin to appear. Dozensof fereral programs encourage and, in some cases, directly subsidize development in places that aren't well-suited to it. Then the natural change becomes erosion. Construction to control it is expensive, ultimately futile and often unexpectedly harmful to the nearby shoreline. Once development reaches a certain level -- Miami Beach, for example -- almost any price can be paid,even though the most massive project may be washed away in the next storm. The constant repair of bridges and beaches on less developed islands is made at the mainland taxpayers' expense.

A recent study found about one-fourthof the federal agencies that administer programs affecting the barrier islands have positive effects on them. One-half of the programs have adverse effects. The remaining quarter are insurance and relief programs that indirectly damage the islands by encouraging development where there would otherwise be none. Without federal flood and disaster insurance programs, for example, mortgage money for much of the building on the islands would be unavailable.

Whether you object to federal money's being wasted or object to the sight of irreplaceable natural systems' being destroyed, the answer to a more sensible barrier island polciy is the same. Undeveloped islands in the barrier island chain should be added to the national park system through a long-term acquisition program.