The Mississippi Delta has always been defined by the sediment flow of the river and level of the ocean. Of these two, sediment flow is less important than ocean level – ocean level has varied more than 200 feet over geologic time. With the ocean level rising, the sediment level does affect how fast the delta is inundated. With a full sediment load and no levees, ocean rise would not submerge the delta as quickly. (It is important to remember that the delta was already receding before man started building levees and dams.) But the Mississippi has been leveed along most of its length, limiting both the sediment going into the river and the ability of the river to deposit this sediment over the delta during floods. Dams have also been built on the upper river and on some feeder streams, further trapping sediment. Without these levees, however, much of the delta and the banks of the upper river would be flooded regularly. This would make it impossible for large cities like New Orleans and St. Louis to exist in their current form. This table gives a good view of the periodic floods of the Mississippi:
http://www.srh.noaa.gov/lix/?n=ms_flood_history