Wetland loss and the subdelta life cycle

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Abstract

The rapid deterioration of marsh habitat observed during recent years in the modern Mississippi River Delta is a consequence, at least in part, of the natural life cycle of subdeltas. With life spans typically less than 200 years, subdeltas or bay-fill deposits are scaled-down versions of major delta lobes, yet provide, through pulses of sediment, nearly all the subaerial land in an active delta. Using maps, charts, and aerial photographs, curves were constructed for rates of change in land area, sediment volume, and linear progradation in the four subdeltas that have formed on the modern Mississippi River Delta since the first accurate survey in 1838.

Results indicate that each subdelta (1) lasted for approximately 115–175 years, (2) included both periods of growth and deterioration, (3) was initiated by a crevasse or break in the natural levee system, (4) showed linear advancement and volumetric growth during subaerial deterioration, and (5) displayed a new pulse of subaerial growth during the high discharge decade of the 1970s. Contrary to popular accounts, demise of the Mississippi River Delta through deterioration of its subdeltas is not a result of the construction of artificial levees upstream or discharge of sediment off the continental shelf edge. Rather, it is attributable to a substantial decrease and fining of sediments being transported downstream to depositional sites within a delta that has developed, through natural processes, a complex and inefficient channel network for delivering these sediments.

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