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.
Some scientists believe that change in sea level has the most important long-term effect on barrier islands. A continuing rise in sea level is the major reason that barrier islands move towards the mainland as time passes. During the past 50 years, sea levels have risen worldwide at an accelerating rate, believed to be partly due to the greenhouse effect. The burning of fossil fuels, cutting of forests, and other human activities are trapping more heat in the Earth's atmosphere. One result is an increased melting of glaciers and the polar ice caps, thus contributing to rising sea levels.