Barrier Islands Explained: Shifting Sand That Shields the Coast
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How we wrote this: A plain-English summary of NOAA's National Ocean Service page "What is a barrier island?", checked October 29, 2026.
Moving piles of sand
A barrier island is a constantly changing deposit of sand that forms parallel to the coast. Waves keep dropping sediment along the shoreline. As wind and waves shift, the islands move, erode and grow, and some disappear entirely.
Two sides of an island
- Ocean side: beaches and sand dunes.
- Mainland side: marshes, tidal flats and maritime forests, separated from the mainland by tidal creeks, bays and lagoons.
These areas are important habitat for seabirds, fish, shellfish and nesting sea turtles.
Where they're found
Many line the Gulf and East coasts, such as North Carolina's Outer Banks. They tend to form near river deltas, in areas with low tectonic activity and flat coastal terrain. The West Coast's rocky cliffs and shifting plates, among other factors, keep them from forming there.
Storm protection
Dunes and beach grasses absorb wave energy before it reaches the mainland. That generally means smaller storm surge and less flooding on the coast; see storm surge and waves.
Under threat
NOAA says barrier islands are disappearing at an alarming rate:
- Human activity causes erosion, including ship traffic, hard levees and projects far upriver.
- Dams and dredging can cut off the sand supply.
- Sea level rise and extreme weather can cause or worsen erosion.
NOAA supports restoration projects, including Gulf Spill Restoration work after Deepwater Horizon.
Related: how beach sand forms, what is an estuary, sea turtle nesting beaches.