The presently accepted theory is that sediments carried by the rising sea level, along with submersion of beach ridges and headlands, made the first barrier islands. The ridges and headlands were eroded by wave action, forming spits or bars and lengthening the spits or bars. The spits or bars protected the lagoon behind them, allowing marshes to form. During storms or periods of high water, inlets formed, which provided nutrients and sediments for the new marshes. Crashing waves may also cause temporary suspension of sediments, which can then be carried along the beach by currents.
Waves are most active on the ocean side of the island but also occur on the bay side.
Strong currents, known as rip currents, run away from the beach back to the ocean and may also carry sediments out to sea. Tidal currents may carry large amounts of sediments through inlets and deposit them in the lagoon or marshes. They may also flood low areas and deposit sediments in marshes or tidal flats. Onshore winds blow sediments from the beach, helping to form dunes. Winds carry sediments from dunes to marshes or bays.