JIM LEHRER: The Mississippi River flood played havoc with commerce today, while towns and cities in two states waited for worse to come.
Ray Suarez has our story.
RAY SUAREZ: By late Tuesday, cargo barges were once again plying the great river near Natchez, Miss. But they were moving at a snail's pace and just one at a time to avoid creating wakes that would put even more pressure on levees.
For most of Tuesday, the Coast Guard suspended traffic on a 15-mile stretch of the river at Natchez. And downstream, 10 freight terminals between Baton Rouge and New Orleans were closed due to high water.
DAVID DELOACH, Deloach Marine Services: We're having to reduce tow size and increase horsepower both on the Mississippi River system and the intercostal canal.
RAY SUAREZ: Indeed, the river is a major artery for carrying goods north and south from the Midwest to the Gulf of Mexico. Normally, up to 600 barges a day move up and down the Mississippi, each carrying as much cargo as 70 tractor-trailers.
The director of the Port of New Orleans warned today that flooding disruptions could cut operations to half capacity. And that, he said, could cost the U.S. economy up to $350 million a day.
Meanwhile, the long wait for the damaging deluge continues in Louisiana's Cajun country.
PIERRE MISTROT, Louisiana: All we just can hope and pay that the dryness of the South will prevail and absorb, hopefully, 100 percent of the water.
RAY SUAREZ: Water levels in some areas have gone down, soaked up by soil parched from drought. On Tuesday, dark green algae line marks near the town of Krotz Springs showed where the water had been on Monday.
Still, people in the town continued to sandbag in preparation. And low-lying parts of Francisville, La., and other small towns were already underwater.
Upriver, in Vicksburg, Miss., the flood tide has forced more than 2,000 people from their homes.
MARY MILLER, Vicksburg, Miss.: It's devastating. It really is. It's heartbreaking and devastating to see your house like that. But I know God didn't bring us this far to leave us, and it's going to be all right.
RAY SUAREZ: The Mississippi is projected to crest tomorrow at Vicksburg, but Gov. Haley Barbour warned today that's not the end of it. He said it could be late June before the water retreats from parts of the Mississippi Delta.
GWEN IFILL: The massive flow of water headed down the Mississippi River is doing more than just flooding neighborhoods. It's also sweeping potentially toxic chemicals into the environment.
NewsHour correspondent Tom Bearden reports from southern Louisiana.
TOM BEARDEN: An enormous quantity of water is surging through the Bonnet Carre Spillway west of New Orleans. It's being diverted from the main channel of the Mississippi River, and flowing into Lake Pontchartrain.
The Corps of Engineers opened the floodgates on May 9 to lower the level of the flood-swollen river and take pressure off the levees that protect New Orleans. But some people are worried about what's in all that the water, like high levels of nitrogen fertilizer, pesticides from flooded fields further north and sediment. And they're wondering how all of that will affect the environment.
We went along with hydrologist Dennis Demcheck and his U.S. Geological Survey team on one of their trips out onto the busy 24-mile-long causeway that bisects Lake Pontchartrain.
DENNIS DEMCHECK, U.S. Geological Survey: So, right now, what I'm going to do is, I'm going to get some profiles of temperature, dissolved oxygen and salinity.
TOM BEARDEN: They're taking water samples to test for things like pesticides and petrochemicals. They're also looking for the level of salt in the water.
DENNIS DEMCHECK: The salinity, it is about 1.5. Normally, it would be double or triple that.
TOM BEARDEN: Pontchartrain is really an enormous lagoon connected to the Gulf, and normally, the water is brackish, a mixture of fresh and sea water.
DENNIS DEMCHECK: For the next couple of months, the character of the lake is going to change. It's going to go from a brackish water lake to a freshwater lake. And so the fish -- some of the fish that are normally in the lake, the saltwater fish -- they are prized by sportsmen -- they're going to swim away. They're not going to die, but they will swim away. And they will come back.
TOM BEARDEN: Demcheck says another concern is that all that fertilizer will provide food for a huge algae bloom later this summer. That will deplete the oxygen in the water.
DENNIS DEMCHECK: So, there could be a minor or transient dead zone, really, oxygen depletion at the bottom of the lake. The saving grace for this is that the lake will flush itself out eventually.
JOHN LOPEZ, Lake Pontchartrain Basin Foundation: It will probably take at least a year or two, some of the impacts.
TOM BEARDEN: John Lopez runs an environmental organization called the Lake Pontchartrain Basin Foundation. He says past algae blooms led to health warnings.
JOHN LOPEZ: During the summer, we will start to see some algal bloom in the lake. Those algal bloom can be -- have some health risks associated with them. They can produce neurotoxins. So, this is something we would monitor over the summer.
TOM BEARDEN: A lot of refineries along the river. Petrochemicals in the water?
JOHN LOPEZ: Yes, there are always some, benzene and some other volatile compounds, almost certainly. These are -- once again, they are diluted. The -- some of them are volatile, so they would be evaporated. The high discharge in the river right now is basically diluting those normal industrial pollutants. So, we don't see those of a concern as long as those plants are working properly within current regulations.
RICHARD CAMPANELLA, Tulane University: We're at 17 now. Typical for this time of year would be eight or nine feet.
TOM BEARDEN: Richard Campanella is a professor at Tulane University, where he teaches about interactions between people and the environment.
RICHARD CAMPANELLA: I think it's convenient to think of this Mississippi River flooding in the system right now as having both benefits and costs.
TOM BEARDEN: He says short-term losses, like the thousands of acres of crops that will be flooded, will be mitigated by long-term gains.
RICHARD CAMPANELLA: It's a jolt to the system. So, the crops that are growing there right now are probably going to be lost. There might be a little stability in next year's crop. But, in the long run, make no mistake that this is very rich sediment that gets deposited here. And again, this is how this entire region was built, and the areas that were built up by highest, closest to these flood-prone freshwater waterways is the most arable land.
TOM BEARDEN: That sediment helps build up land that had been lost to coastal erosion.
RICHARD CAMPANELLA: The river built this land. It built it with flood cycles and sediment. So, here you have a flood cycle getting that water out of the river. And it also pushes back that saltwater wedge and in certain areas builds new land. And this is a good thing.
TOM BEARDEN: But Brad Robin sees no benefits at all.
BRAD ROBIN, oyster fisherman: Fat, healthy.
TOM BEARDEN: His family has been in the oyster business since 1947.
BRAD ROBIN: Delicious.
TOM BEARDEN: Oyster boats are still docking in some of the tiny ports in Saint Bernard Parish southeast of New Orleans, but already two fishing areas have been closed because of the flooding.
BRAD ROBIN: We did not have no time to move any crop we had, because we never thought that they were going to shut it down that fast. And so now we're here, sitting here, waiting. We're praying for the best.
TOM BEARDEN: He says Hurricane Katrina six years ago killed huge swathes of oyster beds. He says they were just beginning to recover when BP's Macondo oil well blew out last spring, and that killed 90 percent of his oyster beds.
BRAD ROBIN: What is coming down that river? We don't know.
TOM BEARDEN: Robin believes that the fresh Mississippi River water that will flow out of Lake Pontchartrain and over his remaining beds will destroy what little is left.
BRAD ROBIN: In the oyster business, we need both. We need -- too much saltwater, the oysters die. Too much freshwater, oysters die.
TOM BEARDEN: And it's not just the water itself.
BRAD ROBIN: The problem with what's coming down the river is more sand, silt that comes in and settles on top of everything. And then we have got to go out there and move it out. It will kill the oysters within two weeks.
TOM BEARDEN: The state has asked the federal government to declare a fisheries disaster and send aid.
BRAD ROBIN: Hurricane Katrina was bad, but we recovered. We haven't recovered from BP. And now this here is just, you know, saying, where we go next? I ask myself, do I think it's worth it? I really don't know.
TOM BEARDEN: Many scientists believe Louisiana's marine ecosystem will recover in just a few years, and ultimately will be healthier for the experience. But Robin says he thinks it will take five years or longer for his oyster beds to come back, and he's not sure the industry can survive that long.