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Building of the Atchafalaya delta constitutes one of the most significant geologic 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 and clay deposition in Atchafalaya Bay and initiated the subaqueous phase of delta building. This development stage 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 extensive subaqueous and subaerial growth during years of major floods. This trend is supported by subaerial transect measurements, which reveal maximum bar aggradation of 0.44 m and up to a 40% reduction in channel cross-sectional area due to levee migration and mid-channel bar formation during floods. In addition, major floods serve to repair lobes eroded during severe winter cold-front passages.
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 adjacent channels. Major channels, separating large lobes, supply sediment to areas bayward of the existing lobes. As the bars coalesce, the distance from the river mouth to the head of the emerging bar decreases and the bifurcation angle increases.
Retreat of this part of the Louisiana coast has occurred since the Bayou Black depositional phase of the Lafourche delta lobe, 1,000 to 2,000 years ago. The Atchafalaya delta, prograding at a maximum rate of 6.5 sq km/year, is helping offset the 42.2 sq km/year loss of Louisiana wetlands.
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