History of N.O. is longer than I wanted to read at http://www.madere.com/ which does not work any more, but if you are really interested google.com a history of New orleans, some is below.

...........The key to securing the Mississippi was to control access to its mouth on the Gulf of Mexico, but the French explorers discovered there was a problem. From the mouth of the Mississippi to a point about 200 miles upstream (Baton Rouge), there was no ground high enough to provide a natural site for a city. While the great river demanded a splendid port city, it seemed to provide no place for one.
................ Early mariners were forced, then, to ask three basic questions as to where their vessels could go and where a city could be located. The basic questions were: Was there a way for deep-water vessels to reach the river other than by entering its mouth far out in the Gulf? Was there a sheltered way through the delta for coastal vessels to avoid the open waters of the Gulf? Was there a place in the delta's featureless slimy muck where goods could be unloaded and stored without risk of frequent flooding? The answer to each question, the French found, was "yes," but a tentative yes................