Delta Switching: Early Responses to the Atchafalaya River Diversion

Abstract

Repeated shifting of the locus of deltaic deposition (delta switching) has been the fundamental process by which the complex delta plain of the Mississippi River has been built. The latest in a series of major Holocene diversions of the river has taken place down the Atchafalaya River course. From the diversion point, this course is approximately 300 km shorter to the Gulf of Mexico than the present Mississippi River course, producing an obvious gradient advantage. Even though control structures presently limit flow down the Atchafalaya to 30% of the Mississippi plus the Red River contribution, dramatic changes have occurred along the central Louisiana coast since Richard J. Russell, James P. Morgan, and their colleagues first reported, in the early 1950s, impressive shoreline adjustments related to the introduction of Atchafalaya sediment. To date, sediment delivered by the Atchafalaya River has (a) filled the Atchafalaya Basin to capacity with lacustrine deltas and swamp deposits; (b) created two bay-head deltas in Atchafalaya Bay (> 150 km of new land above the -0.6 m isobath); and, (c) started a new progradational chapter in the history of the downdrift chenier plain. Since the early 1950s when investigators documented the first accretion along the eastern chenier plain coast, the whole of Atchafalaya Bay, surrounding marshlands, adjacent shelf, and downdrift coasts have experienced a dynamic influx of sediment as a product of the latest Holocene delta switching event. 2