If Huck Finn could taste it now. THE SALTY MISSISSIPPI

In the clear shallow waters bordering this small town 40 miles southeast of New Orleans, prized redfish can be seen shifting slowly in the sun, while crabs skitter sideways across the sandy bottom. Both species inhabit salt water, so sighting them more than 60 miles up the Mississippi River from the open waters of the Gulf of Mexico - and in a river that is usually a churning khaki brown - would in normal times make for a game of ``What's wrong with this picture?''

But this is no normal year on the ``mighty'' Mississippi, whose flow is down nearly two-thirds in the wake of the nation's worst drought in half a century. Without the irresistible thrust of the Mississippi's usually swift current, the Gulf's salty waters have traveled upriver, bringing challenges for the people and industries that line the river's banks in towns like Pointe a la Hache.

Fresh water is being barged in every day from farther upriver, while sales of bottled water boom. But that is only part of the picture.

The Army Corps of Engineers, in a battle to keep the salty flow from reaching New Orleans, last week completed a unique underwater dam, or ``sill,'' to help slow the salt water's advance. The sill - 1,400 feet long, 600 feet wide at its base, and 35 feet tall - is expected to work because salt water, being heavier than fresh water, travels along the river bottom in a wedge pointed upstream.

While most residents and public officials along the river are worried about the salt content of water reaching homes and businesses, environmentalists warn that the already grave erosion of Louisiana's wetlands is accelerating with the invasion of the salt water, and that the river's slow flow means that more toxins are remaining in the river where New Orleans and other cities draw their water.

And even while the Corps of Engineers celebrates what appears by initial tests to be the successful functioning of the sill, some engineers acquainted with the river warn that worse saltwater problems could be coming, since the Mississippi's normal low flow occurs in late summer and fall. Many people believe that, between environmental factors and dredging to open the Mississippi to ever-deeper shipping vessels, the problem of salt intrusion into water supplies is here to stay.

``I would say we will have saltwater intrusion every year now below Belle Chasse sometime between August and November,'' says Luke Petrovich, president of the Plaquemines Parish Council. So far the problems resulting from the salty Mississippi have been concentrated in Plaquemines, a narrow delta of cedar swamps, citrus groves, oil fields, and oyster beds which protrudes 130 miles into the Gulf. Virtually all of the parish's 28,000 residents, plus thousands more from visiting fishing vessels and offshore oil rigs, get their water from the river.

The Corps of Engineers is barging 3 million gallons of fresh water daily to the parish's water plants, where it is mixed with the more saline river water and then sent through normal purification channels. But barging fresh water, while serving parish needs now, is not a long-term solution, according to Mr. Petrovich. The land through which the Mississippi flows, already below sea level here, continues to subside, he notes, while Earth's much-heralded ``greenhouse effect'' is likely over time to raise the level of the Gulf.

A pipeline running fresh water to the lower reaches of the parish is the only long-term solution, says Petrovich, who has served on the council since 1961. ``This is the mouth of the greatest river system in the world, providing billions of dollars to the nation and the state,'' he says. ``I'd say it's in everyone's interest to find answers to our problems.''

Plaquemines residents seem resigned to their water's salinity, having known for several years that a Corps of Engineers plan to dredge the river for deeper vessels would lead to saltwater intrusion.

``We knew it was coming when they decided to dredge the river deeper; we just didn't know it would be this soon,'' says Mike Pursley, a Plaquemines Parish waterworks employee. Standing on a 300,000-gallon barge as water is pumped into a water purification plant here, Mr. Pursley says tap-water salinity was noticeable before the barging began.

``But now it's more rusty than salty,'' he says. ``I guess that's from the barges.'' While some of his neighbors have stopped drinking tap water, he knows of no one planning to move away because of the salinity. ``Most people down here have called this home all their life,'' he says, ``so they aren't going anywhere.''

The Corps of Engineers finished deepening the Mississippi's Southwest Pass, at the tip of Plaquemines Parish, last December. At that time plans for both the salinity sill and freshwater barging were already in place. What was not planned for was this summer's drought.

Corps officials say that the sill, built of sand, clay, and silt dredged from the river bottom, has already worked. The tip of the saltwater wedge had reached New Orleans (though not the city's water intake pumps) in mid-July, but it has since retreated a few miles.

What steps will be taken if the Mississippi's flow continues to fall, however, are unclear.

``If the sill appeared to be inadequate, we would of course look for other measures,'' says Fred Chatry, chief of the corps's engineering division in New Orleans. ``Raising the sill further is one idea, but that would begin to encroach on navigation,'' he notes. ``Ships couldn't load as deep, and that would have serious economic implications.''

Another option would be for municipalities and industries reached by the saltwater wedge to ``water-skim,'' or take their water off the top of the river, instead of from the normal depth of 25 to 30 feet, according to Raphael Kazmann, professor emeritus and former associate director of the Louisiana Water Institute at Louisiana State University.

``But if it doesn't rain long and hard [in the Mississippi watershed] over the normal low-flow months, there will be some real disruptions,'' says Mr. Kazmann; ``you can count on it.''

Virtually everyone agrees that barging fresh water to the New Orleans area's 1.5 million residents is not economically feasible. Yet while some cities in the area are exploring alternatives such as wells, most just seem to be hoping that the sill will continue to function and drastic measures will not be necessary.

Environmentalists worry that impending problems for New Orleans could encourage a diversion to the Mississippi of water from the Atchafalaya River, which empties into the Gulf west of the Mississippi Delta and is the only river system in the United States that is building delta land. Although the Mississippi, under natural conditions, would have shifted course to the Atchafalaya long ago, the Corps of Engineers is required to send 70 percent of the river down the current Mississippi channel.

Mr. Chatry says diversion proposals have been rejected and are unlikely to resurface seriously because the corps recognizes that any benefits would not outweigh damage to the Atchafalaya swamps.

Back in Plaquemines, Luke Petrovich lists the importance of his parish - as home to huge oil and gas operations, one of the greatest seafood-producing regions on the world, and one of the nation's most important shipping routes - and says he is confident solutions will be found.

``The saltwater intrusion problem is just a nit that we can pick out without any trouble,'' he says, ``when there is so much at stake.''