Approximately 30% of the lower Mississippi River presently is diverted into the Atchafalaya distributary. As a consequence, lacustrine deltas rapidly are filling Atchafalaya basin lakes, increasingly coarse-grained sediments are entering Atchafalaya Bay, and a new delta--though probably geologically short-lived--is beginning to form. Understanding the geologic and hydrologic processes giving rise to this newly born delta might provide a clearer insight into the evolution of deltas as recorded in ancient sediments.

In the initial subaqueous phase of deltaic development (between 1952 and 1962) more than 120 sq km of Atchafalaya Bay had been covered by at least 0.5 m of new sediment. Local filling near the delta apex exceeded 2 m. The following phase, an estimated 50-year period of rapid subaerial expansion and shoreline accretion, has just begun. Comparison with modern Mississippi River subdeltas suggests the Atchafalaya delta eventually will deteriorate because of subsidence, compaction, and probable abandonment of the lower river course for a more direct, higher gradient route to the sea.

Analysis of tide records from Eugene Island and other Louisiana coastal stations indicates that in the last 30 years the rate of sea-level rise in Atchafalaya Bay ranged from 0.80 to 1.32 cm/year, almost exclusively because of regional subsidence. This exceeds even the rapid glacio-eustatic sea-level rise 6,000-10,000 years ago--an estimated 0.07 cm/year. Despite deposition into this rapidly subsiding trough, the Atchafalaya delta is still prograding; its ultimate internal form will reflect an interaction of sediment supply, wave energy, and regional tectonism. Unless modified by man, the Atchafalaya delta will expand across its bay 14.2 to 16.9 sq km/year until about the year 2020, creating approximately 950 sq km of new coastal land.

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