THE INFLUENCE OF STORMS AND STRATIGRAPHY ON BARRIER ISLAND EVOLUTION: GULF ISLANDS NATIONAL SEASHORE, GULF OF MEXICO

The barrier islands of the Gulf Islands National Seashore (GUIS) comprise the longest national seashore in the United States. The beaches, dunes, and shoal platforms of the island system stretch from Florida to Mississippi and support a wide variety of vegetation and wildlife. The Mississippi segment of GUIS is presently composed of five islands (Cat, West Ship, East Ship, Horn, and Petit Bois) that support very dynamic littoral systems. Comparative shoreline analysis of maps spanning the past 150 years has documented a net western migration accompanied by episodes of development, destruction, and segmentation. Subsurface analysis reveals a highly variable stratigraphy of Pleistocene fluvial deposits overlain by Holocene nearshore marine deposits. These older deposits are differentially eroded by storm and littoral processes and, in turn, influence the modern morphologic evolution of the islands.

Rates of shoreline loss over the past 60 years have increased. As with other barrier islands in the Gulf of Mexico, reduced sediment supply, storm-impacts, human alteration of the littoral system, and sea level rise have contributed to their deterioration. In 2005, Hurricane Katrina inundated the islands with a 9-m storm surge that expanded existing breaches, destroyed dunes and forests, and scoured the island margins. Individually, however, the islands are experiencing change at widely different rates, and the role of the underlying stratigraphy on this variability is not well constrained. Since 2007, the U.S. Geological Survey (USGS) in collaboration with the National Park Service, has been mapping the seafloor and substrate around the islands as part of the USGS Northern Gulf of Mexico Ecosystem Change and Hazard Susceptibility project. The purpose of these investigations is to characterize the near-surface stratigraphy and identify the influence it may have on island evolution and fate. This presentation highlights recent findings of these studies, and identifies correlations between the Pleistocene-Holocene stratigraphy and island/inlet morphology.