Only source I can post is live streaming video on WWL last night. They were interviewing a bigwig from the Corp of Engineers and the President of one of the parishes south and east of NO. I sat up and took notice at their assessment.
"This is the big economic hit - temporary cessation of major shipping - perhaps almost all shipping - through the Mississippi Delta to the Gulf of Mexico. A truly gargantuan proportion of our GDP depends upon this.Plus loss of the Delta underground/undersea pipelines for crude oil and refined petroleum products due to earth and mud movements. That was a significant problem with the last major hurricane through this area. The problem is likely bigger now, but rebuilding of those pipelines will be slowed by higher priority demands due to all the other damage this time.
But the shipping issue is the biggie."
It is normal for the Mississippi mouth to be temporarily closed to ocean-going ships following a hurricane in the area. The Corps of Engineers has to make certain the main shipping channels remain clear, and to remove new obstructions or dredge new mud flows as necessary.
So Katrina will have some effect here no matter what. But the longer-duration closure you mention would be disastrous.
And the Port of New Orleans is closed indefinitely. It is the port where most foreign cargo is transferred to and from Mississippi River barges to ocean-going shipping. It is much cheaper to move a lot of cargo up and down the river on barges than on ocean-going ships so, AFAIK, almost half the value of exports from the U.S. to foreign markets which passes down the Mississippi, and at least half the value of non-oil/POL imports going up the Mississippi, relies on the Port of New Orleans to get from barges to ships and vice-versa.
Things will be bad enough with the Port of New Orleans closed, though other river ports can probably make up for some of it, but closure of the entire Mississippi mouth for ALL sea-going traffic for significantly longer than the usual post-hurricane period would have a major, major, adverse effect on the U.S. economy.