Most reports explain the traffic slowdown and occasional shutdown as driven by the need to keep wakes down, because ripples can overtop the levees and start erosion. Right now the river level at New Orleans's Carrollton Gauge is 17.23 feet, four feet below the record and up a bit from yesterday.
Not mentioned as prominently in news reports, but equally worrisome, is the danger that a vessel will bash a hole through a levee. That would be particularly ironic to the New Orleans Hyatt, which is to reopen in October after being closed ever since Katrina. The water is so high that in some locations a ship's hull could override the bank and strike a levee. This can happen if a ship loses power or if a barge breaks loose from a tow following a collision or wire break.
During the New Orleans flood of 2005, a storm surge traveling up the Industrial Canal threw a barge against a levee, helping to flood the Lower Ninth Ward. This from the Wall Street Journal, September 8, 2005:
"As storms approach New Orleans, owners of ships, tugboats and freight barges that populate the city's port and waterways attempt to secure their craft.... As the hurricane rolled into New Orleans, scores of boats broke free or sank. In the Industrial Canal, the gush of water broke a barge from its moorings. It isn't known whose barge it was. The huge steel hull became a water-borne missile."
Other allisions of interest:
In 1996 the outbound MV Bright Field lost power while laden with 56,000 tons of corn and drifted into the Riverwalk Marketplace, injuring dozens of mall-goers. Here tugs are taking it away from the scene:
An NTSB report identified a clogged oil filter, and unheeded engine alarms, as the main causes. (As word-lovers know, the verb for crashing into a stationary object is allide, not collide.) Here's a link to a short video clip of the crash.
In 1998 and again in 2004, St. Louis saw runaway barges breaking loose and threatening other vessels along the riverbank. This academic paper looked at the lower Mississippi River and identified stretches most likely to have collisions, allisions, and groundings.
This recent report from the Times-Picayune described risks from tied-up barges and tugs:
"Barges and tugboats are required to stay 180 feet from the levees, for fear that they will crash into the banks and cause damage. But after the Coast Guard prohibited ships from navigating the river at night because of the flooding, barges and tugboats began mooring themselves to the levees so they would not have to keep their engines running at night … .Inspectors on their night-time rounds are on the lookout for the ships. They found nine tied up on Tuesday night and seven on Monday, including some triple-wide barges that could have caused massive damage if they slammed into the levees … Some were close enough that inspectors standing on top of the levees could grab the side of the boat."