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Systematic measurements and comparisons of maps, black-and-white aerial photographs, and color infrared imagery taken at five periods within the interval from 1890 to 1978 have been used to document land loss and habitat change within the Mississippi River deltaic plain. The studies show that the long-term trend of net progradation, which persisted through most of the past 5,000 years, was reversed during the late 19th century, and that during the 20th century land loss rates have accelerated geometrically. Within the 11,500 mi2 (29,900 km2) study area, land loss rates have progressed from approximately 6.7 mi2/year in 1913 to a projected 39.4 mi2/year in 1980. The greatest loss has occurred in the wetlands, but barrier islands nd natural levee ridges are also disappearing at a very high rate.
The data can be used not only to document past change, but also to project future conditions. The findings have great significance to fish and wildlife resources, flood-protection planning, and land ownership.
Apparent causes of the high rates of land loss include the harnessing of the Mississippi River by levees and control structures which reduce tendencies toward natural diversion and funnel valuable sediments to deep, offshore waters. Additional factors include canal dredging and accelerated subsidence
related to mineral extraction, both of which are commonly associated with salt-water intrusion. The net effect is a rapidly accelerating man-induced transgression of a major coastal system.
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