Tidal range-the difference in height between consecutive high and low waters-is an important factor in barrier island formation and morphology. Barriers along coasts with a tidal range of less than 6 ft are classified as microtidal, and those on coasts with a tidal range between 6 and 12 feet are known as mesotidal. Barriers are not usually found along macrotidal coasts (tidal range greater than 12 ft).
Tides also affect inlets between barrier spits and islands. Inlets formed by storms on microtidal barriers are often ephemeral unless structurally stabilized. Conversely, the larger tide of mesotidal coasts creates more stable inlets on mesotidal barriers because of the continual exchange of water between the ocean and back bays.
Tides move sediment through inlets and lead to the formation of flood- and ebb-tidal deltas. Flood-tidal deltas provide new substrate for marsh formation, thereby aiding a barrier's migration.
Lastly, tides dictate the formation of marshes and tidal flats along the bay side of barriers. Low marshes are flooded frequently, while high marshes are found in the intertidal zone and are only flooded at spring high tides and storms. Tidal influences result in the dominance of salt-tolerant plants in marshes.
Microtidal and mesotidal conditions are not the only determining factor of barrier morphology. Waves, sediment supply, inlet stabilization, and tidal prism (the total amount of water that flows into and out of an inlet or harbor with the tide) are critical variables for the shape and movement of barriers.