When the channels widen, land mass is removed and saltwater intrudes into saline intolerant freshwater marsh. Subsidence is caused from geologic faulting and folding, a rising sea level and from soil compaction. As a result of subsidence, the sea is moving inward, causing saline water to encroach further inland. Subsidence has caused a slow, but measurable, loss of freshwater marsh. Land gains and losses become more complex when humans begin to alter the landscape.
Land is gained by filling in marshes for agriculture, development and road construction.
Land loss has occurred by damming the Mississippi River to prevent it from changing course. No new delta lobes are created. The levies, built for coastal storm protection, also prevent flooding from the river, which reduces sediment deposition through overland flow. Subsidence occurs as a result of ground water, oil and gas extraction. Subsidence is also a product of soil compaction from road construction.