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Subsidence in the wetlands near Ironton, Louisianafrom 1932 to 2009

The following set of images illustrates the effects of subsidence on the marsh surface near Ironton, Louisiana. Each page

includes an image taken from the online publication LosingGroundby ProPublica and The Lens on August 28, 2014. The

images were constructed from aerial photographs and Landsat satellite images for the publication to show the changes in the

marsh surface over time. Each page also includes a reconstruction of a profile of core borings that was published by the

U.S.G.S. in 2009 as Open File Report 2009-1158 RecentSubsidence and Erosion at Diverse Wetland Sites in the Southeastern

Mississippi Delta Plain

On each page the profile has been adjusted to the configuration that it would have been in at that time to show how

subsidence formed the open body of water that exists there today. It is important to note that subsidence in this area has

probably been ongoing for thousands of years. As long as the river was in a natural flood cycle, adequate sediments would

have been delivered to maintain the elevation of the marsh even though subsidence was sinking the land surface. As soon

as the sediment supply was cut off, the subsidence caused a change in elevation of the marsh surface, but it took until the

mid-1970s for that subsidence to submerge the marsh surface.

Subsidence has continued until today, and the size of the bay being formed has continued to grow. The suspended sediment

load of the river is a small fraction of what is was when the river was able to maintain the marsh elevation, and there is no

hope that an artificial diversion of the river could rebuild the subsided marsh surface. Any small patches of land that may be

built by a diversion would be immediate subjected to the effects of subsidence. The same is true for the marshcreation

projects that have been attempted in the area.

Based on the study of other hotspots of subsidence in the area, it is reasonable to infer that the subsidence being

experience here is due to the vertical movement of active faults. The rate of subsidence in these hotspots can be estimated

to be in the range of several inches up to 1 foot per decade.

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1932

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1956

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1973

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1975

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1977

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1985

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1988

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1990

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1995

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1998

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2002

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2004

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2006

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2008

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2009

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