Before its western sector was stranded and/or buried ca. 4.0–3.8 ka BP (3.9–3.7 ka 14C), the Mississippi–Alabama chain of regressive barrier islands extended well into present southeastern Louisiana. Westward-directed net littoral drift, ebb-deltas, and microtidal inlet bypassing were instrumental in the formation of elongated, narrow, sandy barrier platform sectors on which these islands, mostly of strandplain topography, have originally emerged. The development of sizable subtidal–intertidal berm basins, ringed by swash and foreshore berm ridges that emerged after storms, then filled by storm-mobilized sand, has aided posthurricane recovery. These processes are linked to discrete stages in aggradational barrier genesis. Increasingly frequent and destructive cyclones reduced island areas to laterally extensive subtidal barrier platform intervals. Enhanced overwash across lengthened platform sectors reduced drift volumes and consequently island progradation. Deepened ship channels facilitated sand loss from littoral drift to offshore seafloor areas. This and the apparent reduction in the longshore sand flux point to natural and human interference with the drift supply. Comparisons of charts, aerial photos, and satellite images provide a quantitative record for the dynamic changes that occurred. Abrupt widening of Petit Bois Pass in 1916 and periodic island diminution and attrition episodes during at least nine hurricanes since had a decisive impact on all the islands. Breaches across low-lying central and eastern (updrift) sectors contributed to long-term island reduction. Starting with Hurricane Betsy (1965), more frequent and destructive tropical cyclones resulted in accelerated island diminution. Damage from wind, salt toxicity, and overwash, combined with shore retreat, seriously impaired the vegetation of several islands in 2005. Because of extensive low and narrow island sectors, Ship and Petit Bois were the most vulnerable. Between 1848–49 and 2005, they suffered 66% and 52% area loss, respectively. Despite recurring but limited post-storm recovery, East Ship now may be approaching extinction. Despite its higher relict beach ridges, secondary dunes, and historically substantial downdrift progradation, even Horn Island has undergone considerable attrition (23%).
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Vol. 2008 • No. 242