It has to worry about rising waters from both directions, both upstream from the Mississippi and off-shore from the tropics.
Here’s a graph showing year-to-date discharge near its mouth, at Tarbert Landing, Mississippi (at boundary of Louisianna).
To date, around 250 million acre-feet has discharged through, and down into the Gulf. That’s the highest year-to-date volume since 1997, which was about 20 million acre-feet more.
To compensate, a portion of the main stems flows are being diverted through Lake Pontchartrain (see article below).
The big flood year for the Mississippi was 1993, when around 550 million acre-feet discharged through Tarbert Landing for the full year.
The average volume from Jan 1st through mid May, since 1970, is right around 200 million acre-feet.
That means that 2008’s half-year flow total is around 50 million acre-feet above the long-term average.
In south Florida terms, that the equivalent of an 13 Lake Okeechobee being poured into the river.
That’s assuming the Lake is fully flooded up to the outermost edges of the littoral zone (around 4 million acre feet).
Of course the Lake is currently holding about half that capacity: 2 million acre feet.
Here’s two interesting articles on New Orleans that highlight its precarious coastal foot-hold in the midst of its upstream and off-shore water forces.