Louisiana's Wetlands Are Sinking Under Pressure
Subsidence — the word has a heavy, technical and unfamiliar sound, but in fact, the term simply refers to land that is sinking. In Louisiana’s coastal wet-lands this phenomenon has occurred for thousands of years, always being offset by new accumulations of soil. The result was a cycle with the following components:
- Spring flooding delivered sediments and nutrients from the Mississippi River to the marshes.
- River sediments settled to the marsh floor.
- Plants grew rapidly, decayed and added to the soil at the bottom of the marsh.
- The weight of the accumulated layers of soil compacted the earth below making way for the process to begin again.
That cycle, however, has been broken. Levees, built to protect the land from floods, keep water out of the marshes, depriving them of sediments and nutrients. Consequently, soil accumulation has slowed significantly – but subsidence has not. The old layers of soil continue to compact. In the nutrient-poor, deepening water, the marsh plants die and ponds develop. Eventually, shallow lakes replace the marsh.
While subsidence is one of the major causes of coastal land loss, wetlands are also affected by other factors, some natural, some man-made. For example, channels and canals built for navigation or oil and gas access draw salt water into the freshwater marshes, while natural erosion occurs on the shore-line of bays and lakes. This combination of forces contributes to the loss of 25 square miles of wetlands each year in coastal Louisiana (see chart above)
Subsidence: with Fresh Water
- Fresh water carries nutrients and sediments from annual floods
- Soil is built from sediments and organic plant material
- Soil compacts
- Level of marsh floor is maintained, water depth remains stable
Sediments from annual flooding and decaying materials allow the floor of a marsh to remain at a constant level.
Subsidence: without Fresh Water
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- Fresh water is blocked
- Soil buildup from sediments and plant materials slows
- Soil compacts, subsidence occurs
- Level of marsh floor drops, water depth increases
- Previous marsh floor
Even when fresh water is absent, compaction continues and subsidence occurs. As a consequence, water depth increases, plants die and ponding results.