BARRIER ISLAND FORMATION: A TALE OF TWO TYPES--COASTAL PLAINS AND DELTAS

A modern survey of barrier island distribution and morphology aided by satellite imagery allows some new insights about the conditions that influence their formation and morphology. Several assumed requirements are revisited: 1) gradient, 2) sediment supply, 3) wave energy, and 4) sea level rise.

Sixty percent of barrier islands are on relatively low-gradient trailing edge continental margins versus only 15% on leading edge margins. The majority of islands on leading edge coasts occur on deltas, whereas drowned estuaries are the most common locations on trailing edges. These two environments lead to genetically different types of barrier islands (deltaic and coastal plain islands). Their relative abundance is also significantly affected by sea level change.

Sediment supply is not necessarily a good predictor of island morphology. Deltaic islands, with typically large supply of sediment, are on average 40% narrower and shorter than coastal plain islands. Deltaic islands are relatively short due to distributary channel spacing. The width of deltaic islands varies depending on the mechanism of island growth.

Wave energy is also a poor predictor of island formation, although a significant factor for island morphology. The low-energy Arctic contains 25% of all barrier islands, but are much shorter and narrower than in any other climate. Arctic islands evolve primarily during storms, the only time when sufficient energy is available.

Sea level rise is essential for most coastal plain islands because drowned river valleys create a highly embayed shoreline that barrier islands gradually straighten and enclose. By far, barrier islands are most abundant in those regions subject to sea level rise—the Atlantic and Arctic coasts of North America and Eurasia—where an overwhelming 95% of islands are of the coastal plain variety. Deltaic islands increase in relative abundance to between 30% and 60% in South America, Africa, South Asia, and Australia.

Deltaic barrier islands do not require sea level rise. Subsidence may be a factor in their formation, as on the Mississippi Delta, however island construction can occur during active progradation. On the Mekong Delta, new islands form as they “jump” seaward of existing islands. These chenier-like islands are narrow, and are also capable of forming during sea level fall.