The Mississippi River in New Orleans is currently at its second lowest flow in recorded history. The previous low was in 1988, at a flow of about 120,000 cfs. It is currently at about 130,000 cfs. In 1988, the salt water wedge made it to the outskirts of New Orleans, but it was of a relatively short duration - about 5 days. Current projects suggest this intrusion may last more like 90 days.
In order to prevent the inflow of salt water, the river needs to be flowing at roughly double its current flow in order to push back against the salt water wedge. To make that happen, the Mississippi River basin as a whole needs about 10" of rain to fall over the entire basin in short order. Thus the predictions of a longer lasting wedge.
Salt water can intrude as the bed of the Mississippi River is at Sea Level around Vicksburg Mississippi (yes, that far up river) - meaning it takes the force of the fresh water flowing to keep it from intruding to that point. To put it in perspective - the elevation of New Orleans is typically stated as about 6 or 7 feet above sea level. The depth of the river there is approximately 200 feet under normal flow conditions.
There are 4 points in past history where the Corps of Engineers has built a sill to attempt to slow down the saltwater wedge intrusion, and they did so this year in July in attempt to slow down the progress of the wedge, so its not correct to say that nothing has been done to slow down or otherwise prepare for the situation at hand. They are working to raise it another 25 feet, but that will only buy days - not weeks. Reality is the drought conditions causing low flows in the river have not abated and in fact increased in severity.
If interested in monitoring the location of the intrusion, the Corps maintains that information:
The estimated cost of bringing in replacement water via pipeline from a point further upriver comes in at around $250 million.
