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Subaqueous Delta Formation—Atchafalaya Bay, Louisiana
About 30 percent of the lower Mississippi River is presently diverted into its Atchafalaya distributary. As a consequence, increasingly coarse-grained sediments are now reaching Atchafalaya Bay, a 520 km2 embayment on the central Louisiana Gulf Coast, and a new marine delta is beginning to form.
Four transitional phases in a delta life cycle will probably develop in the Atchafalaya system: (1) initial flocculation of prodeltaic sediments at the salt water contact resulting in shoreline accretion far removed from the locus of deposition; (2) subaqueous expansion for about 25 years; (3) rapid subaerial progradation and shoreline accretion; and (4) subsidence, compaction, and eventual deterioration of deltaic sublobes.
The subaqueous phase of the present Atchafalaya Delta began about 1952. In the next ten years over 120 km2 of bay bottom had been covered by at least 0.3 m of new sediment. By 1972 sediments derived from coalescing sublobes, emanating from both the lower Atchafalaya River and the Wax Lake Outlet (New Pass), had spread out over 285 km2 of the bay.
Tide gage data from Eugene Island indicate that in the last 30 years there has been an apparent secular rise in sea level averaging 1.3 cm/yr, a rate exceeding even the rapid Holocene glacio-eustatic transgression 10,000 to 6,000 years ago. The sea level rise is apparently due to regional and local subsidence, which has now started to affect the internal form and growth rate of the still generally subaqueous Atchafalaya Delta.
Unless modified even more by man, the Atchafalaya Delta could expand across its bay at an approximate rate of 14 to 17 km2/yr. Like other Holocene-age distributaries of the Mississippi River, the Atchafalaya River will eventually take a new, shorter course to the sea and the delta will slowly subside and deteriorate.
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