The slick has forced the shutdown of the gulf's rich fishing grounds and could also spread to the busy shipping lanes at the mouth of the Mississippi River, tying up the cargo vessels that move millions of tons of fruit, rubber, grain, steel and other commodities and raw materials in and out of the nation's interior.
Though a total shutdown of the shipping lanes is unlikely, there could be long delays if vessels are forced to wait to have their oil-coated hulls power-washed to avoid contaminating the Mississippi.
Some cargo ships might choose to unload somewhere else in the U.S. That could drive up costs.
"Let's say it gets real bad. It gets blocked off and they don't let anything in. They lose time, and they are very concerned about that," said river pilot Michael Lorino. "It's going to be very costly if they have to unload that cargo in another port and ship it back here because it was destined for here."
When a tanker and a tugboat collided near New Orleans two years ago, oil cascaded down the river and some 200 ships stacked up, unable to move for several days while the Coast Guard had the vessels scrubbed. Millions of dollars were lost.
Several river boat pilots said the edge of the oil slick Monday was 15 to 20 miles off the Southwest Pass, where ships headed to New Orleans enter the Mississippi. The latest satellite image of the slick, taken Sunday night, indicates that it has shrunk since last week, but that only means some of the oil has gone underwater.
The new image found oil covering about 2,000 square miles, rather than the roughly 3,400 square miles observed last Thursday, said Hans Graber of the University of Miami.
The new image also shows that sizable patches have broken away and are moving to the north and east, Graber said.
Crews have been struggling to stop the more than 200,000 gallons a day spewing from the sea after an offshore drilling platform blew up and sank last month in a disaster that killed 11 workers. The accident is the worst U.S. oil spill since the tanker Exxon Valdez ran aground in Alaska, leaking nearly 11 million gallons of crude.
Chemical dispersants seemed to be helping to keep oil from floating to the surface, but crews haven't been able to activate a shutout valve underwater. And it could take another week before a 98-ton concrete-and-metal box is placed over one of the leaks to capture the oil.
More ominously, it could take three months to drill sideways into the well and plug it with mud and concrete.
BP said Monday it would compensate people for "legitimate and objectively verifiable" claims from the explosion and spill, but President Barack Obama and others pressed the company to explain exactly what that means.
By all accounts, the disaster is certain to cost BP billions. But analysts said the company could handle it; BP is the world's third-largest oil company and made more than $6 billion in the first three months of this year. The oil spill has drained $32 billion from BP's stock market value.
In the Chandeleur Sound on Monday, about 40 miles northeast of Venice, La., thick, heavy oil formed long clumps that looked like raw sewage. Dying jellyfish could be seen in the water. A dolphin surfaced nearby but did not appear to be in distress.
The news was better from Mississippi to the Florida Panhandle, where the sheen isn't expected to touch beaches before Thursday. Wind and sea currents have helped to keep the oil away from points farther west, said Coast Guard Capt. Steve Poulin.
Poulin used a map that projected what people can expect to see when the oil begins to hit. The outermost layer will include sticky tar balls that can adhere to bird feathers and accumulate in grasses and marshes. The next layer will have bigger pancakes of emulsified oil.
The Port of New Orleans handled 73 millions tons of cargo in 2008, including coffee from South America and steel from Japan, Russia, Brazil and Mexico. More than 245,000 tons of coffee came through the port in 2008, second only to the New York-New Jersey port. And last year, it imported nearly 260,000 tons of rubber from such countries as Indonesia and Malaysia, making it nation's No. 1 gateway for natural rubber.
While a port shutdown would be devastating to the Gulf Coast region, many economists believe the overall damage to the $14.6 trillion U.S. economy could be small.
Cargo can be rerouted to other ports, and the income would be shifted but not lost. Even the lost income from tourism and fishing could be offset by more spending on cleanup.