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The entire delta, under natural conditions, lay above sea level, ranging from a few inches along the coastal fringe to over a dozen feet high at the crest of the Mississippi River’s natural levee.

Throughout New Orleans’ first two centuries, development was densely clustered on this natural levee, making residents less vulnerable to flooding.

Photograph: General Photographic Agency/Getty But massive drainage systems installed in the late-19th and early 20th centuries emptied out city-owned cypress tree swamps behind the natural levee.

Each side of the channel is thus raised so as to form what is termed a natural levee.

The wetlands also act as a natural levee with its grasses and bushes slowing water from moving inland when storms hit the Gulf.