Both the formation and the shape of barrier islands are dependent on tidal inlets and their tidal deltas. The channel of water between adjacent barrier islands is known as an inlet. This is commonly associated with the mouth of a river or stream, or a short-lived feature created when water breaches an island after a large storm such as a hurricane or a Nor'easter. It also provides a regular exchange between the bay and estuarine water with the open ocean. Mesotidal barrier islands, like Folly Island, have the greatest number of inlets due to the strong tidal flow that is capable of maintaining them. However, mesotidal inlets are also subject to migration (Neal, et al, 1984). Inlets are either maintained by tidal currents or closed off by the longshore current.

During a storm surge, large quantities of water are forced into the barrier island's back region. After the storm, the inlet channels the water and sediment back out to sea. If the water level behind the island is higher than sea level, the inlet will be unable to provide drainage to sea and the water will break through at a weak point in the island; thus, creating a new inlet. The "Washout" area on Folly Island is a good example of this event.

The tidal flow deposits sand on either the landward or seaward side of an inlet. The incoming tide delivers a mass of sand on the backside of barrier islands called a flood-tidal delta. These formations allow barrier island systems to migrate landward by widening the back barrier region. The accumulation of sand from the outgoing tide settles out of suspension once the mouth of the inlet reaches the ocean (OCRM, 2000). This forms an ebb-tidal delta on the seaward side of the inlet. Ebb-tidal deltas alter shoreline dynamics by refracting ocean waves.