The Mississippi is America's river, embedded in our history and consciousness. It was the country's first great commercial boulevard, its possibilities immortalized in the literary wanderings of Mark Twain and others. "The face of the water, in time, became a wonderful book . . . delivering its most cherished secrets as clearly as if it uttered them with a voice. And it was not a book to be read once and thrown aside, for it had a new story to tell every day," Twain wrote in Life on the Mississippi. But only in such recollections does the river remain whole. A glimpse of the inexorable Mississippi from a highway bridge or an airliner is still a heart-stopper, but close inspection reveals its forlorn state: polluted, constricted by levees, jammed with barges, backwaters filling with sediment, infested by alien aquatic species, neglected and abused by the cities on its banks.

On the upper river, the U.S. Army Corps of Engineers is scheming to expand the Mississippi's vast commercial navigation system--29 locks and dams from Minneapolis to St. Louis, and another 8 locks and dams on the Illinois, a key tributary. The structures ensure that the shipping channel is deep and wide enough to accommodate towboats and barges, but they hold the water at an artificially high level, turning the upper river into a series of stagnant pools. Without natural ebb and flow, and periodic flooding to rejuvenate backwater marshes and lakes, aquatic life collapses, and floods become more frequent.

What's in that flowing water is just as important to the river's vitality. Huge amounts of sand and other sediment wash into the Mississippi from eroded farm fields and other sources. While some sediment can be beneficial, an excess can smother aquatic plants and fish-spawning beds and eventually fill the backwaters. But at least the silt is natural. Farms and industry throughout the region treat the Mississippi as a handy sewer, and some of what floats by is large enough to have its own sets of eyes. The river has been invaded in recent years by alien aquatic creatures that kill or outcompete native species--some, like the round goby and zebra mussel, from ballast water dumped into the Great Lakes; others, like the bighead carp, from commercial fish ponds.

Conditions in the Mississippi only worsen as you head below St. Louis. There, the river is flanked by the world's largest system of levees, again masterminded by the Corps of Engineers. (Of 25 ongoing Corps projects cited as financially and environmentally wasteful by the National Wildlife Federation and Taxpayers for Common Sense, 9 are on the Mississippi or its tributaries, including a plan to replumb the Mississippi delta with a series of water diversions and channelizations.)

By the time the Mississippi reaches Louisiana, it carries the collected municipal, industrial, and agricultural wastes of the nation's largest river basin--and keeps on going. Beyond the Mississippi delta, the pollutants and nutrients borne by the river have created the infamous Dead Zone, an oxygen-deprived region in the Gulf of Mexico in which fish, crabs, and other aquatic creatures cannot survive. This summer, it grew to the size of Massachusetts.

The only way to repair the Mississippi is to treat the watershed as a whole. Conservationists look at this abused river and imagine it restored to glory: safe for swimming and fishing, reconnected to its floodplain, its natural health given as much consideration as navigability, its sedimentation stemmed, its existence once again celebrated by waterfront cities.

"The Dead Zone isn't occurring in a vacuum," says Doug Daigle of the Mississippi River Basin Alliance, a coalition of community groups that includes the Sierra Club. "Reconnect the Mississippi to its wetlands, then rebuild the marshes, and they will absorb a lot of the nutrients before they enter the Gulf."

Step by step, river-mile by river-mile, that's already occurring. Major soil-conservation programs are under way along two tributaries, the Illinois and the Minnesota, removing flood-prone, environmentally sensitive farmland from crop production and converting it to wetlands, native grasses, and trees. In recent years, experimental drawdowns of water, performed by the Corps of Engineers with the encouragement of environmentalists, have improved conditions for aquatic plants and, in turn, for fish and wildlife. Cities like Minneapolis and Memphis are avidly reclaiming their riverfronts. And on the beleaguered lower river, a consortium called the Lower Mississippi Conservation Committee has developed a plan for improving water quality and restoring habitat by reconnecting the river to floodplain marshes and other backwaters without interfering with commercial navigation.

"At what point does the Mississippi become what the Mississippi means?" St. Louis-raised T. S. Eliot asked in an essay about the river. Ask river activists what the answer should be and they'll say, "Everywhere."