At times much of the island is covered in marshes. The marshes are only temporary and are referred to as “ephemeral marshes”. They form from rainwater or extreme tidal events during the wetter seasons and may stay for several months. Often the marshes are completely dry for a few months at a time. Because the water may be only a few inches deep while the grass is often over a foot high, unless cattails are growing in them, the marshes are often hidden from view. The marshes are the main source of fresh water for wildlife on the island. Other than the marshes there are only three permanent freshwater ponds in the park, which are located in the northernmost ten miles of the park.