Lessons from the collapse of Mississippi delta marsh wetland

Flooding of Mississippi River Delta from the Gulf of Mexico -- due to 
levees that divert sediment, extreme weather conditions, 
and freshwater canals which prevent the deposition 
of sediment which is the primary mechanism by which the 
delta was originally formed and is replenished.

What happens when you 'over-engineer' and do not take a simple thing like sediment deposition into consideration?  How does 'land' disappear?

"Subsidence" is the gradual sinking or caving in of land caused by a number of things including: climatic, mechanical [physical], chemical and other processes such as:

• lowering of the water table due to withdrawals [agriculture-irrigation, mining, salt removal, oil, natural gas extraction, etc.] which creates a space . . . and water will fill it.
• obstruction of the natural deposition of sediment due to re-routing of sediment-laden rivers and streams by levees, seawalls, etc. which cut sediment supply off from its original destination.
• chemical dissolution of limestone [composed mainly of calcium carbonate] which comes down in rain [the atmosphere absorbs carbon dioxide resulting in carbonic acid] with water filling in cracks, holes, caves, etc.
• hydrocompaction of soils when organic-rich sediment [peat, etc.] is subjected to loss of water due to draining of swamps.   

In marshes, deltas, etc. subsidence is offset by the deposit of sediment. Coastal wetland sedimentation is the key thing in keeping marshlands alive.  "By holding in floodwaters, the levees prevent sediment delivery onto much of the river's natural floodplain," said Carol B. Lutken, associate director of the Mississippi Mineral Resources Institute at the University of Mississippi.  "The levees focus greater volumes of water over a diminished area which results in more of the flow arriving at the river's mouth.  This focusing of flow enables the river to carry a greater suspended load of sediment and increases the proportion that will bypass coastal marshes."

Scientists agree: "The Mississippi River Delta is disappearing at an astonishing rate: A football field of wetlands vanishes into open water almost every hour. Since the 1930s, Louisiana has lost nearly 1,900 square miles of land, an area roughly equivalent in size to the state of Delaware.  Many factors have led to the delta's collapse. One of the most significant is that the lower Mississippi River has been straitjacketed with huge levees as part of a national program to 'control' the Mississippi River and protect communities, economic infrastructure and croplands from river flooding. But the delta's wetlands are built and sustained by sediment delivered by the river. Cutting the river off from its delta with levees doomed existing wetlands and largely stopped the cycle of new wetlands growth. Without land-building deposits from the river, the delta is doomed to continue sinking beneath the water, endangering people, wildlife and jobs," says the organization Restore the Mississippi River Delta.