ABSTRACT

Building of the Atchafalaya Delta constitutes one of the most significant geological events in historical times within the Mississippi Delta complex. Periodic upstream diversions, such as the present Atchafalaya River, result in switching of the major loci of active deposition and are among the fundamental mechanisms of Mississippi delta growth.

Prior to 1950, Atchafalaya sediment was trapped in intrabasin lakes and swamps. Thereafter, progressive basin filling prompted silt/clay deposition in Atchafalaya Bay and initiated the subaqueous phase of delta building. This developmental stage started in the early 1950's and continued until the appearance of sand-dominant subaerial lobes in 1972, after which rapid subaerial growth occurred.

Development of the Atchafalaya Delta is related to major flood pulses of the Mississippi River. Interpretation of LANDSAT imagery and aerial photography indicates subaerial growth during years of major floods. Distributary channels experienced up to 40 percent reduction in cross-sectional area owing to levee migration and mid-channel bar formation during floods.

River-mouth processes are frictionally dominated. Channel-mouth bifurcation, accompanied by coarse-particle deposition, is the major process of lobe initiation. Larger lobes are the result of coalescence of numerous distributary-mouth bars and channel-fill sequences. Major channels, separating large lobes, supply sediment to areas bayward of the subaerial delta.

The Atchafalaya Delta represents an entirely new prograding sand body in an area of the Mississippi deltaic plain which over the past several thousand years has been characterized by slow deposition of fine-grained sediments and coastal retreat.